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(C) 深圳瓶子科技有限公司 版权所有 粤B2-汽车电动机启动时车灯会瞬时变暗.如图24-5.在打开车灯的情况下.电动机未启动时电流表读数为10 A.电动机启动时电流表读数为58 A.若电源电动势为12.5 V.内阻为0.05 Ω.电流表内阻不计.则因电动机启动.车灯的电功率降低了( ) 图24-5 A．35.8 W B．43.2 W C．48.2 W D．76.8 W 题目和参考答案——精英家教网——
暑假天气热？在家里学北京名师课程，
& 题目详情
汽车电动机启动时车灯会瞬时变暗，如图24－5，在打开车灯的情况下，电动机未启动时电流表读数为10 A，电动机启动时电流表读数为58 A，若电源电动势为12.5 V，内阻为0.05 Ω，电流表内阻不计，则因电动机启动，车灯的电功率降低了(　　)
图24－5 A．35.8 W& &&&&&&&&&&&&&&&&&&&&&&&&&&&& B．43.2 W C．48.2 W& &&&&&&&&&&&&&&&&&&&&&&&&&&&& D．76.8 W
解析：电动机未启动时，灯泡两端电压UL＝E－I1r＝12.5 V－10×0.05 V＝12 V．灯泡电阻RL＝＝1.2 Ω，灯泡功率P1＝ULI1＝120 W．电动机启动时，I2＝58 A，灯泡两端电压U′L＝E－I2r＝9.6 V，灯泡功率P2＝＝ W＝76.8 W，车灯的功率降低了ΔP＝P1－P2＝43.2 W. 答案：B
练习册系列答案
科目：高中物理
图18－10 如图18－10所示，物体A的质量为M，圆环B的质量为m，通过绳子连接在一起，圆环套在光滑的竖直杆上，开始时连接圆环的绳子处于水平，长度l＝4 m，现从静止释放圆环．不计定滑轮和空气的阻力，取g＝10 m/s2，求： (1)为使圆环能下降h＝3 m，两个物体的质量应满足什么关系？ (2)若圆环下降h＝3 m时的速度v＝5 m/s，则两个物体的质量有何关系？ (3)不管两个物体的质量为多大，圆环下降h＝3 m时的速度不可能超过多大？
科目：高中物理
某导体置于电场后周围的电场分布情况如图22－9所示，图中虚线表示电场线，实线表示等势面，A、B、C为电场中的三个点．下列说法错误的是(　　)
图22－9 A．A点的电场强度小于B点的电场强度 B．A点的电势高于B点的电势 C．将负电荷从A点移到B点，电场力做正功 D．将正电荷从A点移到C点，电场力做功为零
科目：高中物理
反射式速调管是常用的微波器件之一，它利用电子团在电场中的振荡来产生微波，其振荡原理与下述过程类似．如图23－11所示，在虚线MN两侧分别存在着方向相反的两个匀强电场，一带电微粒从A点由静止开始，在电场力作用下沿直线在A、B两点间往返运动．已知电场强度的大小分别是E1＝2.0×103 N/C和E2＝4.0×103 N/C，方向如图23－11所示，带电微粒质量m＝1.0×10－20 kg，带电量q＝－1.0×10－9C，A点距虚线MN的距离d1＝1.0 cm，不计带电微粒的重力，忽略相对论效应．求：
图23－11 (1)B点距虚线MN的距离d2； (2)带电微粒从A点运动到B点所经历的时间t.
科目：高中物理
功率为10 W的发光二极管(LED灯)的亮度与功率为60 W的白炽灯相当．根据国家节能战略，2016年前普通白炽灯应被淘汰．假设每户家庭有2只60 W的白炽灯，均用10& W的LED灯替代，估算出全国一年节省的电能最接近(　　) A．8×108 kW·h& &&&&&&&&&&&&&&&&&&&&&& B．8×1010 kW·h C．8×1011 kW·h& &&&&&&&&&&&&&&&&&&&&&& D．8×1013 kW·h
科目：高中物理
电源的电动势和内电阻都保持一定，在外电路的电阻逐渐减小的过程中，下面说法正确的是(　　) A．电源的路端电压一定逐渐变小 B．电源的输出功率一定逐渐变小 C．电源内部消耗的功率一定逐渐变大 D．电源的供电效率一定逐渐变小
科目：高中物理
如图25－7所示，直线Ⅰ、Ⅱ分别是电源1与电源2的路端电压随输出电流变化的特性图线，曲线Ⅲ是一个小灯泡的伏安特性曲线，如果把该小灯泡先后分别与电源1和电源2单独连接时，则下列说法不正确的是(　　)
图25－7 A．电源1和电源2的内阻之比是11∶7 B．电源1和电源2的电动势之比是1∶1 C．在这两种连接状态下，小灯泡消耗的功率之比是1∶2 D．在这两种连接状态下，小灯泡的电阻之比是1∶2
科目：高中物理
图27－5为多用表欧姆挡的原理示意图，其中电流表的满偏电流为300 μA，
图27－5 内阻rg＝100 Ω，调零电阻最大阻值R＝50 kΩ，串联的固定电阻R0＝50 Ω，电池电动势E＝1.5 V，用它测量电阻Rx，能准确测量的阻值范围是(　　) A．30～80 kΩ B．3～8 kΩ C．300～800 kΩ D．3 000～8 000 kΩ
科目：高中物理
如图30－12所示，在y轴右方有一垂直纸面向里的匀强磁场，磁感应强度B＝0.20 T．有一质子以速度v＝2.0×106 m/s，从x轴上的A点沿与x轴正方向成30°角斜向上射入磁场．质子质量m＝1.6×10－27 kg，电荷量q＝1.6×10－19 C，质子重力不计．求：
图30－12 (1)质子在磁场中做圆周运动的半径； (2)若质子能沿x轴负方向通过y轴上的C点，如图30－12所示，求C 点离坐标原点的距离． (3)在y轴左侧存在水平向右的匀强电场，场强大小E＝105 N/C，求质子从A点出发至速度第一次为零所用的时间．(结果保留两位有效数字)
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perlvar - Perl predefined variables
DESCRIPTION
The Syntax of Variable Names
Variable names in Perl can have several formats.
Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
In this case, the part before the last ::
' is taken to be a package qualifier; see .
A Unicode letter that is not ASCII is not considered to be a letter
unless &use utf8&
is in effect, and somewhat more complicated
for details.
Perl variable names may also be a sequence of digits, a single
punctuation character, or the two-character sequence: ^ (caret or
CIRCUMFLEX ACCENT) followed by any one of the characters [][A-Z^_?\]
These names are all reserved for
special uses by P for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
preceded by a caret.
These must all be written in the form ${^Foo}
the braces are not optional.
denotes the scalar variable
whose name is considered to be a control-F
followed by two o
These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (caret-underscore).
name that begins with ^_ will acquire a special
meaning in any future version of P such names may therefore be
used safely in programs.
itself, however, is reserved.
Perl identifiers that begin with digits or
punctuation characters are exempt from the effects of the
declaration and are always forced to be in package main
; they are
also exempt from strict 'vars'
A few other names are also
exempt in these ways:
SIGIn particular, the special ${^_XYZ}
variables are always taken
to be in package main
, regardless of any
declarations
presently in scope.
SPECIAL VARIABLES
The following names have special meaning to Perl.
Most punctuation
names have reasonable mnemonics, or analogs in the shells.
Nevertheless, if you wish to use long variable names, you need only say:
English;at the top of your program.
This aliases all the short names to the long
names in the current package.
Some even have medium names, generally
borrowed from awk.
For more info, please see .
Before you continue, note the sort order for variables.
In general, we
first list the variables in case-insensitive, almost-lexigraphical
order (ignoring the { or ^ preceding words, as in ${^UNICODE}
), although $_
move up to the top of the pile.
For variables with the same identifier, we list it in order of scalar,
array, hash, and bareword.
General Variables
The default input and pattern-searching space.
The following pairs are
equivalent:
(&&) {...}
# equivalent only in while!
(($_ = &&)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
($_)Here are the places where Perl will assume $_
even if you don't use it:
The following functions use $_
as a default argument:
abs, alarm, chomp, chop, chr, chroot,
cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
rmdir, say, sin, split (for its second
argument), sqrt, stat, study, uc, ucfirst,
unlink, unpack.
All file tests (-f
) except for -t
, which defaults to STDIN.
The pattern matching operations ,
when used without an =~
The default iterator variable in a foreach
loop if no other
variable is supplied.
The implicit iterator variable in the
functions.
The implicit variable of given()
The default place to put the next value or input record
when a &FH&
operation's result is tested by itself as the sole criterion of a while
Outside a while
test, this will not happen.
is by default a global variable.
However, as
of perl v5.10.0, you can use a lexical version of
by declaring it in a file or in a block with .
restores the global $_
in the current scope.
this seemed like a good idea at the time it was introduced, lexical $_
actually causes more problems than it solves.
If you call a function that
expects to be passed information via $_
, it may or may not work,
depending on how the function is written, there not being any easy way to
solve this.
Just avoid lexical $_
, unless you are feeling particularly
masochistic.
For this reason lexical $_
is still experimental and will
produce a warning unless warnings have been disabled.
As with other
experimental features, the behavior of lexical $_
is subject to change
without notice, including change into a fatal error.
Mnemonic: underline is understood in certain operations.
Within a subroutine the array @_
contains the parameters passed to
that subroutine.
Inside a subroutine, @_
is the default array for
the array operators
$LIST_SEPARATOR
When an array or an array slice is interpolated into a double-quoted
string or a similar context such as /.../
, its elements are
separated by this value.
Default is a space.
For example, this:
&The array is: @array\n&;is equivalent to this:
&The array is: & . ($&, @array) . &\n&;Mnemonic: works in double-quoted context.
$PROCESS_ID
The process number of the Perl running this script.
Though you can set
this variable, doing so is generally discouraged, although it can be
invaluable for some testing purposes.
It will be reset automatically
Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
would emulate POSIX semantics on Linux systems using LinuxThreads, a
partial implementation of POSIX Threads that has since been superseded
by the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching getpid()
like this made embedding perl unnecessarily complex (since you'd have
to manually update the value of $$), so now $$
will always return the same values as the underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and
including the 6.0 release, but after that moved to FreeBSD thread
semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if
getconf GNU_LIBPTHREAD_VERSION |
returns a false
NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.
$PROGRAM_NAME
Contains the name of the program being executed.
On some (but not all) operating systems assigning to $0
the argument area that the ps
program sees.
On some platforms you
may have to use special ps
options or a different ps
to see the
Modifying the $0
is more useful as a way of indicating the
current program state than it is for hiding the program you're
Note that there are platform-specific limitations on the maximum
length of $0
In the most extreme case it may be limited to the
space occupied by the original $0
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by ps
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).
Note for BSD users: setting $0
does not completely remove "perl"
from the ps(1) output.
For example, setting $0
to &foobar&
result in &perl: foobar (perl)&
(whether both the &perl: &
and the " (perl)" suffix are shown depends on your exact BSD variant
and version).
This is an operating system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the $0
and the change becomes visible
to ps(1) (assuming the operating system plays along).
the view of $0
the other threads have will not change since they
have their own copies of it.
If the program has been given to perl via the switches -e
will contain the string &-e&
On Linux as of perl v5.14.0 the legacy process name will be set with
, in addition to altering the POSIX name via argv[0]
perl has done since version 4.000.
Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to $0
The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.
Mnemonic: same as sh and ksh.
$REAL_GROUP_ID
The real gid of this process.
If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in.
The first number is the one returned by
, and the subsequent ones by getgroups()
, one of which may be
the same as the first number.
However, a value assigned to $(
must be a single number used to
set the real gid.
So the value given by $(
should not be assigned
back to $(
without being forced numeric, such as by adding zero.
that this is different to the effective gid ($)
) which does take a
You can change both the real gid and the effective gid at the same
time by using POSIX::setgid()
require a check to $!
to detect any possible errors after an attempted change.
Mnemonic: parentheses are used to group things.
The real gid is the
group you left, if you're running setgid.
$EFFECTIVE_GROUP_ID
The effective gid of this process.
If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in.
The first number is the one
returned by getegid()
, and the subsequent ones by getgroups()
one of which may be the same as the first number.
Similarly, a value assigned to $)
must also be a space-separated
list of numbers.
The first number sets the effective gid, and
the rest (if any) are passed to setgroups()
To get the effect of an
empty list for setgroups()
, just repeat t that is,
to force an effective gid of 5 and an effectively empty setgroups()
$) = &5 5&
You can change both the effective gid and the real gid at the same
time by using POSIX::setgid()
(use only a single numeric argument).
Changes to $)
require a check to $!
to detect any possible errors
after an attempted change.
can be set only on
machines that support the corresponding set[re][ug]id() routine.
can be swapped only on machines supporting setregid()
Mnemonic: parentheses are used to group things.
The effective gid
is the group that's right for you, if you're running setgid.
$REAL_USER_ID
The real uid of this process.
You can change both the real uid and the
effective uid at the same time by using POSIX::setuid()
changes to $&
require a system call, check $!
after a change
attempt to detect any possible errors.
Mnemonic: it's the uid you came from, if you're running setuid.
$EFFECTIVE_USER_ID
The effective uid of this process.
For example:
# set real to effective uid
($&,$&) = ($&,$&);
# swap real and effective uidsYou can change both the effective uid and the real uid at the same
time by using POSIX::setuid()
Changes to $&
require a check
to detect any possible errors after an attempted change.
can be swapped only on machines
supporting setreuid()
Mnemonic: it's the uid you went to, if you're running setuid.
$SUBSCRIPT_SEPARATOR
The subscript separator for multidimensional array emulation.
refer to a hash element as
$foo{$x,$y,$z}it really means
$foo{($;, $x, $y, $z)}But don't put
@foo{$x,$y,$z} # a slice--note the @which means
($foo{$x},$foo{$y},$foo{$z})Default is "\034", the same as SUBSEP in awk.
If your keys contain
binary data there might not be any safe value for $;
Consider using "real" multidimensional arrays as described
Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
Special package variables when using , see .
Because of this specialness $a
don't need to be declared
, or ) even when using the strict 'vars'
Don't lexicalize them with
if you want to
be able to use them in the
comparison block or function.
The hash %ENV
contains your current environment.
value in ENV
changes the environment for any child processes
you subsequently
As of v5.18.0, both keys and values stored in %ENV
are stringified.
$ENV{'bar'} = \$foo;
$ENV{'bar'} ) {
&Pre 5.18.0 Behaviour&;
&Post 5.18.0 Behaviour&;
}Previously, only child processes received stringified values:
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
($^X, '-e',
q/print ( ref $ENV{'bar'}
? 'ref' : 'non ref' ) /);This happens because you can't really share arbitrary data structures with
foreign processes.
$OLD_PERL_VERSION
The revision, version, and subversion of the Perl interpreter, represented
as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
is the subversion / 1e6.
For example, Perl v5.10.1 would be "5.010001".
This variable can be used to determine whether the Perl interpreter
executing a script is in the right range of versions:
&No PerlIO!\n&
'5.008';When comparing $]
, string comparison operators are highly
recommended.
The inherent limitations of binary floating point
representation can sometimes lead to incorrect comparisons for some
numbers on some architectures.
See also the documentation of
for a convenient way to fail if the running Perl interpreter is too old.
for a representation of the Perl version as a
object, which allows more flexible string comparisons.
The main advantage of $]
is that it works the same on any
version of Perl.
The disadvantages are that it can't easily be compared
to versions in other formats (e.g. literal v-strings, "v1.2.3" or
version objects) and numeric comparisons c it's good
for string literal version checks and bad for comparing to a variable
that hasn't been sanity-checked.
The $OLD_PERL_VERSION
form was added in Perl v5.20.0 for historical
reasons but its use is discouraged. (If your reason to use $]
run code on old perls then referring to it as $OLD_PERL_VERSION
be self-defeating.)
Mnemonic: Is this version of perl in the right bracket?
$SYSTEM_FD_MAX
The maximum system file descriptor, ordinarily 2.
System file
descriptors are passed to ed processes, while higher file
descriptors are not.
Also, during an
, system file descriptors are
preserved even if the
fails (ordinary file descriptors are
closed before the
is attempted).
The close-on-exec
status of a file descriptor will be decided according to the value of
when the corresponding file, pipe, or socket was opened, not the
time of the .
The array @F
contains the fields of each line read in when autosplit
mode is turned on.
for the -a switch.
This array
is package-specific, and must be declared or given a full package name
if not in package main when running under strict 'vars'
The array @INC
contains the list of places that the
constructs look for their library files.
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent the current
directory.
("." will not be appended if taint checks are enabled,
either by -T
, or if configured not to do so by the
-Ddefault_inc_excludes_dot
compile time option.)
If you need to
modify this at runtime, you should use the
pragma to get
the machine-dependent library properly loaded also:
lib '/mypath/libdir/';
SomeMod;You can also insert hooks into the file inclusion system by putting Perl
code directly into @INC
Those hooks may be subroutine references,
array references or blessed objects.
for details.
The hash %INC
contains entries for each filename included via the
operators.
The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found.
operator uses this hash to determine whether a particular file has
already been included.
If the file was loaded via a hook (e.g. a subroutine reference, see
for a description of these hooks), this hook is
by default inserted into %INC
in place of a filename.
Note, however,
that the hook may have set the %INC
entry by itself to provide some more
specific info.
$INPLACE_EDIT
The current value of the inplace-edit extension.
to disable
inplace editing.
Mnemonic: value of -i switch.
Each package contains a special array called @ISA
which contains a list
of that class's parent classes, if any. This array is simply a list of
scalars, each of which is a string that corresponds to a package name. The
array is examined when Perl does method resolution, which is covered in
To load packages while adding them to @ISA
pragma. The
discouraged
pragma does this as well, but should not be used except
when compatibility with the discouraged
pragma is required.
By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of $^M
as an emergency memory pool after ing.
Suppose that your Perl
were compiled with -DPERL_EMERGENCY_SBRK
and used Perl's malloc.
$^M = 'a' x (1 && 16);would allocate a 64K buffer for use in an emergency.
INSTALL file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl.
To discourage casual
use of this advanced feature, there is no
long name for
this variable.
This variable was added in Perl 5.004.
The name of the operating system under which this copy of Perl was
built, as determined during the configuration process.
For examples
The value is identical to $Config{'osname'}
and the -V command-line switch documented in .
In Windows platforms, $^O
is not very helpful: since it is always
, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET.
Use Win32::GetOSName()
Win32::GetOSVersion() (see
and ) to distinguish
between the variants.
This variable was added in Perl 5.003.
The hash %SIG
contains signal handlers for signals.
For example:
sub handler {
# 1st argument is signal name ($sig) = @_;
&Caught a SIG$sig--shutting down\n&; (LOG); (0); }
$SIG{'INT'}
= \&handler;
$SIG{'QUIT'} = \&handler;
$SIG{'INT'}
= 'DEFAULT';
# restore default action
$SIG{'QUIT'} = 'IGNORE';
# ignore SIGQUITUsing a value of 'IGNORE'
usually has the effect of ignoring the
signal, except for the CHLD
for more about
this special case.
Here are some other examples:
$SIG{&PIPE&} = &Plumber&;
# assumes main::Plumber (not
# recommended)
$SIG{&PIPE&} = \&Plumber;
# assume current
$SIG{&PIPE&} = *Plumber;
# somewhat esoteric
$SIG{&PIPE&} = Plumber();
# oops, what did Plumber()
# return??Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.
If your system has the sigaction()
function then signal handlers
are installed using it.
This means you get reliable signal handling.
The default delivery policy of signals changed in Perl v5.8.0 from
immediate (also known as "unsafe") to deferred, also known as "safe
for more information.
Certain internal hooks can be also set using the %SIG
routine indicated by $SIG{__WARN__}
is called when a warning
message is about to be printed.
The warning message is passed as the
first argument.
The presence of a __WARN__
hook causes the
ordinary printing of warnings to STDERR
to be suppressed.
use this to save warnings in a variable, or turn warnings into fatal
errors, like this:
$SIG{__WARN__} =
$proggie;As the 'IGNORE'
hook is not supported by __WARN__
disable warnings using the empty subroutine:
$SIG{__WARN__} =
{};The routine indicated by $SIG{__DIE__}
is called when a fatal
exception is about to be thrown.
The error message is passed as the
first argument.
When a __DIE__
hook routine returns, the exception
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a
, a loop exit,
The __DIE__
handler is explicitly disabled during
the call, so that you can die from a __DIE__
for __WARN__
The $SIG{__DIE__}
hook is called even inside an . It was
never intended to happen this way, but an implementation glitch made
this possible. This used to be deprecated, as it allowed strange action
at a distance like rewriting a pending exception in $@
. Plans to
rectify this have been scrapped, as users found that rewriting a
pending exception is actually a useful feature, and not a bug.
handlers are very special in one respect: they
may be called to report (probable) errors found by the parser.
a case the parser may be in inconsistent state, so any attempt to
evaluate Perl code from such a handler will probably result in a
This means that warnings or errors that result from parsing
Perl should be used with extreme caution, like this:
Carp::confess(&Something wrong&)
&Carp::confess;
&Something wrong, but could not load Carp to give &
. &backtrace...\n\t&
. &To see backtrace try starting Perl with -MCarp switch&;Here the first line will load Carp
unless it is the parser who
called the handler.
The second line will print backtrace and die if
was available.
The third line will be executed only if Carp
not available.
Having to even think about the $^S
variable in your exception
handlers is simply wrong.
$SIG{__DIE__}
as currently implemented
invites grievous and difficult to track down errors.
and use an END{}
or CORE::GLOBAL::die override instead.
See , , , and
for additional information.
The time at which the program began running, in seconds since the
epoch (beginning of 1970).
The values returned by the -M, -A,
and -C filetests are based on this value.
$PERL_VERSION
The revision, version, and subversion of the Perl interpreter,
represented as a
This variable first appeared in perl v5.6.0; earlier versions of perl
will see an undefined value.
Before perl v5.10.0 $^V
was represented
as a v-string rather than a
can be used to determine whether the Perl interpreter executing
a script is in the right range of versions.
For example:
&Hashes not randomized!\n& if !$^V or $^V lt v5.8.1While version objects overload stringification, to portably convert
into its string representation, use 's &%vd&
conversion, which works for both v-strings or version objects:
&version is v%vd\n&, $^V;
# Perl's versionSee the documentation of
for a convenient way to fail if the running Perl interpreter is too old.
for a decimal representation of the Perl version.
The main advantage of $^V
is that, for Perl v5.10.0 or
later, it overloads operators, allowing easy comparison against other
version representations (e.g. decimal, literal v-string, "v1.2.3", or
The disadvantage is that prior to v5.10.0, it was only a
literal v-string, which can't be easily printed or compared, whereas
the behavior of $]
is unchanged on all versions of Perl.
Mnemonic: use ^V for a version object.
${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then
on Windows will
not try to open the file.
This means that the link count cannot be
determined and file attributes may be out of date if additional
hardlinks to the file exist.
On the other hand, not opening the file
is considerably faster, especially for files on network drives.
This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy"
See the documentation for -f in
for more information about site
customization.
This variable was added in Perl v5.10.0.
$EXECUTABLE_NAME
The name used to execute the current copy of Perl, from C's
or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
perl program file.
Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
is no guarantee that the value of $^X
is in PATH.
For VMS, the
value may or may not include a version number.
You usually can use the value of $^X
to re-invoke an independent
copy of the same perl that is currently running, e.g.,
@first_run = `$^X -le &print int rand 100 for 1..100&`;But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.
It is not safe to use the value of $^X
as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command.
To convert the value of $^X
to a path name, use the
following statements:
# Build up a set of file names (not command names).
$this_perl = $^X;
'VMS') { $this_perl .= $Config{_exe}
$this_perl =~ m/$Config{_exe}$/i; }Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
copy referenced by $^X
The following statements accomplish
this goal, and produce a pathname that can be invoked as a
command or referenced as a file.
$secure_perl_path = $Config{perlpath};
'VMS') { $secure_perl_path .= $Config{_exe}
$secure_perl_path =~ m/$Config{_exe}$/i; }
Variables related to regular expressions
Most of the special variables related to regular expressions are side
Perl sets these variables when it has a successful match, so
you should check the match result before using them.
For instance:
( /P(A)TT(ER)N/ ) {
&I found $1 and $2\n&; }These variables are read-only and dynamically-scoped, unless we note
otherwise.
The dynamic nature of the regular expression variables means that
their value is limited to the block that they are in, as demonstrated
by this bit of code:
$outer = 'Wallace and Grommit';
$inner = 'Mutt and Jeff';
$pattern = qr/(\S+) and (\S+)/;
sub show_n {
&\$1 is $1; \$2 is $2\n& }
OUTER: show_n()
$outer =~ m/$pattern/; INNER: {
$inner =~ m/$pattern/;
} show_n();
}The output shows that while in the OUTER
block, the values of $1
are from the match against $outer
Inside the INNER
block, the values of $1
are from the match against
, but only until the end of the block (i.e. the dynamic
After the INNER
block completes, the values of $1
return to the values for the match against $outer
even though
we have not made another match:
$1 is Wallace; $2 is Grommit
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is GrommitPerformance issues
Traditionally in Perl, any use of any of the three variables
equivalents) anywhere in the code, caused
all subsequent successful pattern matches to make a copy of the matched
string, in case the code might subsequently access one of those variables.
This imposed a considerable performance penalty across the whole program,
so generally the use of these variables has been discouraged.
In Perl 5.6.0 the @-
dynamic arrays were introduced that
supply the indices of successful matches. So you could for example do
$str =~ /pattern/;
$`, $&, $'; # bad: perfomance hit
# good: no perfomance hit ($str, 0,
$-[0]), ($str, $-[0], $+[0]-$-[0]), ($str, $+[0]);In Perl 5.10.0 the /p match operator flag and the ${^PREMATCH}
, and ${^POSTMATCH}
variables were introduced, that allowed
you to suffer the penalties only on patterns marked with /p.
In Perl 5.18.0 onwards, perl started noting the presence of each of the
three variables separately, and only copied that part of the string
$`; $&; &abcdefgh& =~ /d/perl would only copy the "abcd" part of the string. That could make a big
difference in something like
$str = 'x' x 1_000_000;
$&; # whoops
$str =~ /x/g # one char copied a million times, not a million charsIn Perl 5.20.0 a new copy-on-write system was enabled by default, which
finally fixes all performance issues with these three variables, and makes
them safe to use anywhere.
The Devel::NYTProf
and Devel::FindAmpersand
modules can help you
find uses of these problematic match variables in your code.
$&digits& ($1, $2, ...)
Contains the subpattern from the corresponding set of capturing
parentheses from the last successful pattern match, not counting patterns
matched in nested blocks that have been exited already.
Note there is a distinction between a capture buffer which matches
the empty string a capture buffer which is optional. Eg, (x?)
The latter may be undef, the former not.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
@{^CAPTURE}
An array which exposes the contents of the capture buffers, if any, of
the last successful pattern match, not counting patterns matched
in nested blocks that have been exited already.
Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1 index
is equivalent to $2, etc.
(&foal&=~/(.)(.)(.)(.)/) {
&-&, @{^CAPTURE};
}should output "f-o-a-l".
See also ,
Note that unlike most other regex magic variables there is no single
letter equivalent to @{^CAPTURE}
This variable was added in 5.25.7
The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or
enclosed by the current
above for the serious performance implications
of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: like &
in some editors.
This is similar to $&
) except that it does not incur the
performance penalty associated with that variable.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.
In Perl v5.20, the /p modifier does nothing, so
does the same thing as $MATCH
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
The string preceding whatever was matched by the last successful
pattern match, not counting any matches hidden within a BLOCK or
enclosed by the current BLOCK.
above for the serious performance implications
of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: ` often precedes a quoted string.
${^PREMATCH}
This is similar to $`
($PREMATCH) except that it does not incur the
performance penalty associated with that variable.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.
In Perl v5.20, the /p modifier does nothing, so
${^PREMATCH}
does the same thing as $PREMATCH
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$POSTMATCH
The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or
enclosed by the current BLOCK).
$_ = 'abcdefghi';
&$`:$&:$'\n&;
# prints abc:def:ghiSee
above for the serious performance implications
of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: ' often follows a quoted string.
${^POSTMATCH}
This is similar to $'
($POSTMATCH
) except that it does not incur the
performance penalty associated with that variable.
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.
In Perl v5.20, the /p modifier does nothing, so
${^POSTMATCH}
does the same thing as $POSTMATCH
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$LAST_PAREN_MATCH
The text matched by the last bracket of the last successful search pattern.
This is useful if you don't know which one of a set of alternative patterns
For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
$LAST_SUBMATCH_RESULT
The text matched by the used group most-recently closed (i.e. the group
with the rightmost closing parenthesis) of the last successful search
This is primarily used inside (?{...}) blocks for examining text
recently matched.
For example, to effectively capture text to a variable
(in addition to $1
, etc.), replace (...)
(?:(...)(?{ $var = $^N }))By setting and then using $var
in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
This variable was added in Perl v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.
@LAST_MATCH_END
This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope.
the offset into the string of the end of the entire match.
is the same value as what the
function returns when called
on the variable that was matched against.
The nth element
of this array holds the offset of the nth submatch, so
is the offset past where $1
ends, $+[2]
the offset
past where $2
ends, and so on.
You can use $#+
to determine
how many subgroups were in the last successful match.
examples given for the @-
This variable was added in Perl v5.6.0.
%{^CAPTURE}
%LAST_PAREN_MATCH
Similar to @+
hash allows access to the named capture
buffers, should they exist, in the last successful match in the
currently active dynamic scope.
For example, $+{foo}
is equivalent to $1
after the following match:
'foo' =~ /(?&foo&foo)/;The keys of the %+
hash list only the names of buffers that have
captured (and that are thus associated to defined values).
The underlying behaviour of %+
is provided by the
are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing
iterative access to them via
may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl v5.10.0. The %{^CAPTURE}
added in 5.25.7.
This variable is read-only and dynamically-scoped.
@LAST_MATCH_START
is the offset of the start of the last successful match.
$-[n] is the offset of the start of the substring matched by
n-th subpattern, or undef if the subpattern did not match.
Thus, after a match against $_
coincides with
$_, $-[0],
$+[0] - $-[0]
Similarly, $n coincides with
$_, $-[n],
$+[n] - $-[n]
is defined, and $+ coincides with
$_, $-[$#-], $+[$#-] - $-[$#-]
One can use $#-
to find the
last matched subgroup in the last successful match.
Contrast with
, the number of subgroups in the regular expression.
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
is the offset into the string of the beginning of the
entire match.
The nth element of this array holds the offset
of the nth submatch, so $-[1]
is the offset where $1
begins, $-[2]
the offset where $2
begins, and so on.
After a match against some variable $var
is the same as ($var, 0, $-[0])
is the same as ($var, $-[0], $+[0] - $-[0])
is the same as ($var, $+[0])
is the same as ($var, $-[1], $+[1] - $-[1])
is the same as ($var, $-[2], $+[2] - $-[2])
is the same as ($var, $-[3], $+[3] - $-[3])
This variable was added in Perl v5.6.0.
%{^CAPTURE_ALL}
Similar to %+
, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope.
each capture group name found in the regular expression, it associates a
reference to an array containing the list of values captured by all
buffers with that name (should there be several of them), in the order
where they appear.
Here's an example:
('1234' =~ /(?&A&1)(?&B&2)(?&A&3)(?&B&4)/) {
$bufname (
$ary = $-{$bufname};
$idx (0..$#$ary) {
&\$-{$bufname}[$idx] : &,
(($ary-&[$idx])
? &'$ary-&[$idx]'&
: &undef&),
}would print out:
$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'The keys of the %-
hash correspond to all buffer names found in
the regular expression.
The behaviour of %-
is implemented via the
are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing
iterative access to them via
may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl v5.10.0. The %{^CAPTURE_ALL}
added in 5.25.7.
This variable is read-only and dynamically-scoped.
$LAST_REGEXP_CODE_RESULT
The result of evaluation of the last successful (?{ code })
regular expression assertion (see ).
May be written to.
This variable was added in Perl 5.005.
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags.
Set to 0 for no debug output
even when the re 'debug'
module is loaded.
for details.
This variable was added in Perl v5.10.0.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how much memory they
This value by default is 65536 which corresponds to a 512kB
temporary cache.
Set this to a higher value to trade
memory for speed when matching large alternations.
it to a lower value if you want the optimisations to
be as conservative of memory as possible but still occur, and set it to a
negative value to prevent the optimisation and conserve the most memory.
Under normal situations this variable should be of no interest to you.
This variable was added in Perl v5.10.0.
Variables related to filehandles
Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the IO::Handle
although this is less efficient than using the regular built-in
variables.
(Summary lines below for this contain the word HANDLE.)
First you must say
IO::Handle;after which you may use either
method HANDLE EXPRor more safely,
HANDLE-&method(EXPR)Each method returns the old value of the IO::Handle
attribute.
methods each take an optional EXPR, which, if supplied, specifies the
new value for the IO::Handle
attribute in question.
supplied, most methods do nothing to the current value--except for
autoflush()
, which will assume a 1 for you, just to be different.
Because loading in the IO::Handle
class is an expensive operation,
you should learn how to use the regular built-in variables.
A few of these variables are considered "read-only".
This means that
if you try to assign to this variable, either directly or indirectly
through a reference, you'll raise a run-time exception.
You should be very careful when modifying the default values of most
special variables described in this document.
In most cases you want
to localize these variables before changing them, since if you don't,
the change may affect other modules which rely on the default values
of the special variables that you have changed.
This is one of the
correct ways to read the whole file at once:
$fh, &&&, &foo& or
$/; # enable localized slurp mode
$content = &$fh&;
$fh;But the following code is quite bad:
$fh, &&&, &foo& or
$/; # enable slurp mode
$content = &$fh&;
$fh;since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of $/
is now changed for any other code
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible.
So unless you are already
inside some short {}
block, you should create one yourself.
$content = '';
$fh, &&&, &foo&
$/; $content = &$fh&;
$fh;Here is an example of how your own code can go broken:
( 1..3 ){ $\ = &\r\n&; nasty_break();
sub nasty_break { $\ = &\f&; # do something with $_
}You probably expect this code to print the equivalent of
&1\r\n2\r\n3\r\n&but instead you get:
&1\f2\f3\f&Why? Because nasty_break()
modifies $\
without localizing it
The value you set in
nasty_break()
is still there when you
The fix is to add
so the value doesn't leak out of
nasty_break()
$\ = &\f&;It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
changes to the special variables.
Contains the name of the current file when reading from &&
The array @ARGV
contains the command-line arguments intended for
the script.
is generally the number of arguments minus
one, because $ARGV[0]
is the first argument, not the program's
command name itself.
for the command name.
The special filehandle that iterates over command-line filenames in
Usually written as the null filehandle in the angle operator
Note that currently ARGV
only has its magical effect
within the &&
elsewhere it is just a plain filehandle
corresponding to the last file opened by &&
In particular,
passing \*ARGV
as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in @ARGV
The special filehandle that points to the currently open output file
when doing edit-in-place processing with -i.
Useful when you have
to do a lot of inserting and don't want to keep modifying $_
for the -i switch.
IO::Handle-&output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
The output field separator for the print operator.
If defined, this
value is printed between each of print's arguments.
Default is .
You cannot call output_field_separator()
on a handle, only as a
static method.
Mnemonic: what is printed when there is a "," in your print statement.
HANDLE-&input_line_number( EXPR )
$INPUT_LINE_NUMBER
Current line number for the last filehandle accessed.
Each filehandle in Perl counts the number of lines that have been read
(Depending on the value of $/
, Perl's idea of what
constitutes a line may not match yours.)
When a line is read from a
filehandle (via
), or when
is called on it, $.
becomes an alias to the line counter
for that filehandle.
You can adjust the counter by assigning to $.
, but this will not
actually move the seek pointer.
Localizing $.
will not localize
the filehandle's line count.
Instead, it will localize perl's notion
of which filehandle $.
is currently aliased to.
is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening .
details, see .
Because &&
never does
an explicit close, line numbers increase across ARGV
files (but see
examples in ).
You can also use HANDLE-&input_line_number(EXPR)
to access the
line counter for a given filehandle without having to worry about
which handle you last accessed.
Mnemonic: many programs use "." to mean the current line number.
IO::Handle-&input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
The input record separator, newline by default.
This influences Perl's
idea of what a "line" is.
Works like awk's RS variable, including
treating empty lines as a terminator if set to the null string (an
empty line cannot contain any spaces or tabs).
You may set it to a
multi-character string to match a multi-character terminator, or to
to read through the end of file.
Setting it to &\n\n&
means something slightly different than setting to &&
, if the file
contains consecutive empty lines.
Setting to &&
will treat two or
more consecutive empty lines as a single empty line.
Setting to
will blindly assume that the next input character belongs to
the next paragraph, even if it's a newline.
# enable &slurp& mode
$_ = &FH&;
# whole file now here
s/\n[ \t]+/ /g;Remember: the value of $/
is a string, not a regex.
awk has to
be better for something. :-)
Setting $/
to a reference to an integer, scalar containing an
integer, or scalar that's convertible to an integer will attempt to
read records instead of lines, with the maximum record size being the
referenced integer number of characters.
$/ = \32768; # or \&32768&, or \$var_containing_32768
$fh, &&&, $myfile or
$_ = &$fh&;will read a record of no more than 32768 characters from $fh.
not reading from a record-oriented file (or your OS doesn't have
record-oriented files), then you'll likely get a full chunk of data
with every read.
If a record is larger than the record size you've
set, you'll get the record back in pieces.
Trying to set the record
size to zero or less is deprecated and will cause $/ to have the value
of "undef", which will cause reading in the (rest of the) whole file.
As of 5.19.9 setting $/
to any other form of reference will throw a
fatal exception. This is in preparation for supporting new ways to set
in the future.
On VMS only, record reads bypass PerlIO layers and any associated
buffering, so you must not mix record and non-record reads on the
same filehandle.
Record mode mixes with line mode only when the
same buffering layer is in use for both modes.
You cannot call input_record_separator()
on a handle, only as a
static method.
See also .
Also see .
Mnemonic: / delimits line boundaries when quoting poetry.
IO::Handle-&output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
The output record separator for the print operator.
If defined, this
value is printed after the last of print's arguments.
Default is .
You cannot call output_record_separator()
on a handle, only as a
static method.
Mnemonic: you set $\
instead of adding "\n" at the end of the print.
Also, it's just like $/
, but it's what you get "back" from Perl.
HANDLE-&autoflush( EXPR )
$OUTPUT_AUTOFLUSH
If set to nonzero, forces a flush right away and after every write or
print on the currently selected output channel.
Default is 0
(regardless of whether the channel is really buffered by the system or
tells you only whether you've asked Perl explicitly to
flush after each write).
STDOUT will typically be line buffered if
output is to the terminal and block buffered otherwise.
Setting this
variable is useful primarily when you are outputting to a pipe or
socket, such as when you are running a Perl program under rsh and
want to see the output as it's happening.
This has no effect on input
buffering.
how to select the output channel.
See also .
Mnemonic: when you want your pipes to be piping hot.
${^LAST_FH}
This read-only variable contains a reference to the last-read filehandle.
This is set by &HANDLE&
This is the same handle that $.
without arguments
It is also the handle used when Perl appends ", &STDIN& line 1" to
an error or warning message.
This variable was added in Perl v5.18.0.
Variables related to formats
The special variables for formats are a subset of those for
filehandles.
for more information about Perl's
$ACCUMULATOR
The current value of the
accumulator for
A format contains
calls that put their result into
After calling its format,
prints out the contents
and empties.
So you never really see the contents of $^A
unless you call
yourself and then look at it.
IO::Handle-&format_formfeed(EXPR)
$FORMAT_FORMFEED
What formats output as a form feed.
The default is \f
You cannot call format_formfeed()
on a handle, only as a static
HANDLE-&format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
The current page number of the currently selected output channel.
Mnemonic: %
is page number in nroff.
HANDLE-&format_lines_left(EXPR)
$FORMAT_LINES_LEFT
The number of lines left on the page of the currently selected output
Mnemonic: lines_on_page - lines_printed.
IO::Handle-&format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format.
The default is
" \n-", to break on a space, newline, or a hyphen.
You cannot call format_line_break_characters()
on a handle, only as
a static method.
Mnemonic: a "colon" in poetry is a part of a line.
HANDLE-&format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
The current page length (printable lines) of the currently selected
output channel.
The default is 60.
Mnemonic: = has horizontal lines.
HANDLE-&format_top_name(EXPR)
$FORMAT_TOP_NAME
The name of the current top-of-page format for the currently selected
output channel.
The default is the name of the filehandle with _TOP
For example, the default format top name for the STDOUT
filehandle is STDOUT_TOP
Mnemonic: points to top of page.
HANDLE-&format_name(EXPR)
$FORMAT_NAME
The name of the current report format for the currently selected
output channel.
The default format name is the same as the filehandle
For example, the default format name for the STDOUT
filehandle is just STDOUT
Mnemonic: brother to $^
Error Variables
The variables $@
contain information
about different types of error conditions that may appear during
execution of a Perl program.
The variables are shown ordered by
the "distance" between the subsystem which reported the error and
the Perl process.
They correspond to errors detected by the Perl
interpreter, C library, operating system, or an external program,
respectively.
To illustrate the differences between these variables, consider the
following Perl expression, which uses a single-quoted string.
execution of this statement, perl may have set all four special error
variables:
open my $pipe, &/cdrom/install |& or die $!;
my @res = &$pipe&;
close $pipe or die &bad pipe: $?, $!&;
};When perl executes the
expression, it translates the
calls in the C run-time library
and thence to the operating system kernel.
perl sets $!
the C library's errno
if one of these calls fails.
is set if the string to be -ed did not compile (this may
were imported with bad prototypes), or
if Perl code executed during evaluation d.
In these cases the
value of $@
is the compile error, or the argument to
will interpolate $!
(See also , though.)
Under a few operating systems, $^E
may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed."
Systems that
do not support extended error messages leave $^E
the same as $!
Finally, $?
may be set to a non-0 value if the external program
/cdrom/install fails.
The upper eight bits reflect specific error
conditions encountered by the program (the program's
The lower eight bits reflect mode of failure, like signal death and
core dump information.
See wait(2) for details.
In contrast to
, which are set only if an error condition is detected,
the variable $?
is set on each
overwriting the old value.
This is more like $@
, which on every
is always set on failure and cleared on success.
For more details, see the individual descriptions at $@
${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick (``
command, successful call to
or , or from the
On POSIX-like systems this value can be decoded
with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
WSTOPSIG and WIFCONTINUED functions provided by the
Under VMS this reflects the actual VMS i.e. it is the
same as $?
when the pragma
vmsish 'status'
is in effect.
This variable was added in Perl v5.10.0.
$EXTENDED_OS_ERROR
Error information specific to the current operating system.
moment, this differs from
under only VMS, OS/2, and Win32 (and
for MacPerl).
On all other platforms, $^E
is always just the same
Under VMS, $^E
provides the VMS status value from the last system
This is more specific information about the last system error
than that provided by $!
This is particularly important when $!
is set to EVMSERR.
Under OS/2, $^E
is set to the error code of the last call to OS/2
API either via CRT, or directly from perl.
Under Win32, $^E
always returns the last error information reported
by the Win32 call GetLastError()
which describes the last error
from within the Win32 API.
Most Win32-specific code will report errors
ANSI C and Unix-like calls set errno
and so most
portable Perl code will report errors via $!
Caveats mentioned in the description of
generally apply to
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
$EXCEPTIONS_BEING_CAUGHT
Current state of the interpreter.
State ---------
-------------------------------------
Parsing module, ,
main program true (1)
Executing an
OtherwiseThe first state may happen in $SIG{__DIE__}
and $SIG{__WARN__}
The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
value does not indicate whether exceptions are being caught,
since compilation of the main program does not catch exceptions.
This variable was added in Perl 5.004.
The current value of the warning switch, initially true if -w was
used, false otherwise, but directly modifiable.
See also .
Mnemonic: related to the -w switch.
${^WARNING_BITS}
The current set of warning checks enabled by the
It has the same scoping as the $^H
variables.
values are considered internal to the
pragma and may change
between versions of Perl.
This variable was added in Perl v5.6.0.
When referenced, $!
retrieves the current value
of the C errno
integer variable.
is assigned a numerical value, that value is stored in errno
When referenced as a string, $!
yields the system error string
corresponding to errno
Many system or library calls set errno
if they fail,
to indicate the cause of failure.
They usually do not
to zero if they succeed.
This means errno
, is meaningful only immediately after a failure:
$fh, &&&, $filename) {
# Here $! is meaningless.
# ONLY here is $! meaningful.
# Already here $! might be meaningless.
# Since here we might have either success or failure,
# $! is meaningless.Here, meaningless means that $!
may be unrelated to the outcome
Assignment to $!
is similarly ephemeral.
It can be used immediately before invoking the
to set the exit value, or to inspect the system error string
corresponding to error n, or to restore $!
to a meaningful state.
Mnemonic: What just went bang?
Each element of %!
has a true value only if $!
is set to that
For example, $!{ENOENT}
is true if and only if the current
value of $!
; that is, if the most recent error was "No
such file or directory" (or its moral equivalent: not all operating
systems give that exact error, and certainly not all languages).
specific true value is not guaranteed, but in the past has generally
been the numeric value of $!
To check if a particular key is
meaningful on your system, use
$!{the_key}
; for a list of legal
for more information, and also see
This variable was added in Perl 5.005.
$CHILD_ERROR
The status returned by the last pipe close, backtick (``
) command,
successful call to
or , or from the
This is just the 16-bit status word returned by the
traditional Unix
system call (or else is made up to look
Thus, the exit value of the subprocess is really ($?&&
), and $? & 127
gives which signal, if any, the process died
from, and $? & 128
reports whether there was a core dump.
Additionally, if the h_errno
variable is supported in C, its value
is returned via $?
if any gethost*()
function fails.
If you have installed a signal handler for SIGCHLD
value of $?
will usually be wrong outside that handler.
Inside an END
subroutine $?
contains the value that is going to be
given to .
You can modify $?
subroutine to
change the exit status of your program.
For example:
END { $? = 1 if $? == 255;
# die would make it 255
}Under VMS, the pragma
vmsish 'status'
reflect the
actual VMS exit status, instead of the default emulation of POSIX
for details.
Mnemonic: similar to sh and ksh.
$EVAL_ERROR
The Perl error from the last
operator, i.e. the last exception that
was caught.
, this is either a runtime error message or the
string or reference
was called with.
catches syntax errors and other compile time exceptions.
If no error occurs,
to the empty string.
Warning messages are not collected in this variable.
You can, however,
set up a routine to process warnings by setting $SIG{__WARN__}
described in .
Mnemonic: Where was the error "at"?
Variables related to the interpreter state
These variables provide information about the current interpreter state.
$COMPILING
The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD
at compile
time rather than normal, deferred loading.
is similar to calling B::minus_c
This variable was added in Perl v5.6.0.
$DEBUGGING
The current value of the debugging flags.
May be read or set.
, you can use numeric
or symbolic values, e.g. $^D = 10
or $^D = &st&
The contents of this variable also affects the
debugger operation.
Mnemonic: value of -D switch.
${^ENCODING}
This variable is no longer supported.
It used to hold the object reference to the Encode
object that was
used to convert the source code to Unicode.
Its purpose was to allow your non-ASCII Perl
scripts not to have to be written in UTF-8; this was
useful before editors that worked on UTF-8 encoded text were common, but
that was long ago.
It caused problems, such as affecting the operation
of other modules that weren't expecting it, causing general mayhem.
If you need something like this functionality, it is recommended that use
you a simple source filter, such as .
If you are coming here because code of yours is being adversely affected
by someone's use of this variable, you can usually work around it by
doing this:
${^ENCODING};near the beginning of the functions that are getting broken.
undefines the variable during the scope of execution of the including
This variable was added in Perl 5.8.2 and removed in 5.26.0.
${^GLOBAL_PHASE}
The current phase of the perl interpreter.
Possible values are:
The PerlInterpreter*
is being constructed via perl_construct
value is mostly there for completeness and for use via the
underlying C variable PL_phase
It's not really possible for Perl
code to be executed unless construction of the interpreter is
This is the global compile-time.
That includes, basically, every
block executed directly or indirectly from during the
compile-time of the top-level program.
This phase is not called "BEGIN" to avoid confusion with
-blocks, as those are executed during compile-time of any
compilation unit, not just the top-level program.
A new, localised
compile-time entered at run-time, for example by constructs as
&use SomeModule&
are not global interpreter phases, and
therefore aren't reflected by ${^GLOBAL_PHASE}
Execution of any CHECK
Similar to "CHECK", but for INIT
-blocks, not CHECK
The main run-time, i.e. the execution of PL_main_root
Execution of any END
Global destruction.
Also note that there's no value for UNITCHECK-blocks.
That's because
those are run for each compilation unit individually, and therefore is
not a global interpreter phase.
Not every program has to go through each of the possible phases, but
transition from one phase to another can only happen in the order
described in the above list.
An example of all of the phases Perl code can see:
&compile-time: ${^GLOBAL_PHASE}\n& }
&init-time: ${^GLOBAL_PHASE}\n& }
&check-time: ${^GLOBAL_PHASE}\n& }
package Print::Phase;
($class, $time) = @_;
\$time, $class;
sub DESTROY {
&$$self: ${^GLOBAL_PHASE}\n&;
&run-time: ${^GLOBAL_PHASE}\n&;
$runtime = Print::Phase-&new(
&lexical variables are garbage collected before END&
&end-time: ${^GLOBAL_PHASE}\n& }
$destruct = Print::Phase-&new(
&package variables are garbage collected after END&
);This will print out
compile-: START
check-: CHECK
init-: INIT
lexical variables are garbage collected before END: RUN
package variables are garbage collected after END: DESTRUCTThis variable was added in Perl 5.14.0.
WARNING: This variable is strictly for
internal use only.
Its availability,
behavior, and contents are subject to change without notice.
This variable contains compile-time hints for the Perl interpreter.
end of compilation of a BLOCK the value of this variable is restored to the
value when the interpreter started to compile the BLOCK.
When perl begins to parse any block construct that provides a lexical scope
(e.g., eval body, required file, subroutine body, loop body, or conditional
block), the existing value of $^H
is saved, but its value is left unchanged.
When the compilation of the block is completed, it regains the saved value.
Between the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of $^H
This behavior provides the semantic of lexical scoping, and is used in,
for instance, the
The contents
different bits of it are used for
different pragmatic flags.
Here's an example:
sub add_100 { $^H |= 0x100 }
sub foo { BEGIN { add_100() } bar-&baz($boon);
}Consider what happens during execution of the BEGIN block.
At this point
the BEGIN block has already been compiled, but the body of foo()
being compiled.
The new value of $^H
will therefore be visible only while
the body of foo()
is being compiled.
Substitution of BEGIN { add_100() }
block with:
strict; strict-&import('vars') }demonstrates how
strict 'vars'
is implemented.
Here's a conditional
version of the same lexical pragma:
strict; strict-&import('vars')
$condition
}This variable was added in Perl 5.003.
hash provides the same scoping semantic as $^H
This makes
it useful for implementation of lexically scoped pragmas.
All the entries are stringified when accessed at
runtime, so only simple values can be accommodated.
This means no
pointers to objects, for example.
When putting items into %^H
, in order to avoid conflicting with other
users of the hash there is a convention regarding which keys to use.
A module should use only keys that begin with the module's name (the
name of its main package) and a "/" character.
For example, a module
should use keys such as Foo::Bar/baz
This variable was added in Perl v5.6.0.
An internal variable used by PerlIO.
A string in two parts, separated
byte, the first part describes the input layers, the second
part describes the output layers.
This variable was added in Perl v5.8.0.
The internal variable for debugging support.
The meanings of the
various bits are subject to change, but currently indicate:
Debug subroutine enter/exit.
Line-by-line debugging.
Causes DB::DB()
subroutine to be called for
each statement executed.
Also causes saving source code lines (like
Switch off optimizations.
Preserve more data for future interactive inspections.
Keep info about so