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1=head1 NAME
3perlvar - Perl predefined variables
b0c22438 7=head2 The Syntax of Variable Names
0b9346e6 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
0b9346e6 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
16Perl variable names may also be a sequence of digits or a single
0b9346e6 17punctuation or control character. These names are all reserved for
b0c22438 18special uses by Perl; for example, the all-digits names are used
19to hold data captured by backreferences after a regular expression
0b9346e6 20match. Perl has a special syntax for the single-control-character
b0c22438 21names: It understands C<^X> (caret C<X>) to mean the control-C<X>
0b9346e6 22character. For example, the notation C<$^W> (dollar-sign caret
b0c22438 23C<W>) is the scalar variable whose name is the single character
0b9346e6 24control-C<W>. This is better than typing a literal control-C<W>
b0c22438 25into your program.
27Since Perl 5.6, Perl variable names may be alphanumeric
28strings that begin with control characters (or better yet, a caret).
29These variables must be written in the form C<${^Foo}>; the braces
0b9346e6 30are not optional. C<${^Foo}> denotes the scalar variable whose
31name is a control-C<F> followed by two C<o>'s. These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
0b9346e6 33begin with C<^_> (control-underscore or caret-underscore). No
b0c22438 34control-character name that begins with C<^_> will acquire a special
35meaning in any future version of Perl; such names may therefore be
0b9346e6 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
38Perl identifiers that begin with digits, control characters, or
39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
0b9346e6 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
0b9346e6 44 ENV STDIN
b0c22438 48 SIG
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
a0d0e21e 55
0b9346e6 56The following names have special meaning to Perl. Most punctuation
57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
0b9346e6 60 use English;
a0d0e21e 61
62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
84dabc03 64borrowed from B<awk>. To avoid a performance hit, if you don't need the
65C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
66module without them:
a0d0e21e 67
0b9346e6 68 use English '-no_match_vars';
a1ce9542 69
0b9346e6 70Before you continue, note the sort order for variables. In general, we
71first list the variables in case-insensitive, almost-lexigraphical
72order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74For variables with the same identifier, we list it in order of scalar,
75array, hash, and bareword.
a1ce9542 76
b0c22438 77=head2 General Variables
a0d0e21e 78
84dabc03 79=over 8
81=item $ARG
83=item $_
a054c801 84X<$_> X<$ARG>
a0d0e21e 85
b0c22438 86The default input and pattern-searching space. The following pairs are
0b9346e6 89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
a0d0e21e 91
0b9346e6 92 /^Subject:/
93 $_ =~ /^Subject:/
a0d0e21e 94
0b9346e6 95 tr/a-z/A-Z/
96 $_ =~ tr/a-z/A-Z/
a0d0e21e 97
0b9346e6 98 chomp
99 chomp($_)
a0d0e21e 100
0b9346e6 101Here are the places where Perl will assume C<$_> even if you don't use it:
103=over 3
105=item *
84dabc03 107The following functions use C<$_> as a default argument:
db1511c8 108
109abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
110hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
111quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
b0c18621 112rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
b0169937 113unlink, unpack.
115=item *
117All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
118See L<perlfunc/-X>
120=item *
122The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
123when used without an C<=~> operator.
cb1a09d0 124
54310121 125=item *
127The default iterator variable in a C<foreach> loop if no other
128variable is supplied.
54310121 130=item *
cb1a09d0 131
b0c22438 132The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 133
54310121 134=item *
cb1a09d0 135
b0c22438 136The implicit variable of C<given()>.
138=item *
c47ff5f1 140The default place to put an input record when a C<< <FH> >>
cb1a09d0 141operation's result is tested by itself as the sole criterion of a C<while>
b0c22438 142test. Outside a C<while> test, this will not happen.
59f00321 146As C<$_> is a global variable, this may lead in some cases to unwanted
b0c22438 147side-effects. As of perl 5.9.1, you can now use a lexical version of
148C<$_> by declaring it in a file or in a block with C<my>. Moreover,
4fd88bf8 149declaring C<our $_> restores the global C<$_> in the current scope.
59f00321 150
b0c22438 151Mnemonic: underline is understood in certain operations.
a0d0e21e 152
0b9346e6 153=item @ARG
cde0cee5 154
0b9346e6 155=item @_
156X<@_> X<@ARG>
a0d0e21e 157
0b9346e6 158Within a subroutine the array C<@_> contains the parameters passed to
159that subroutine. Inside a subroutine, C<@_> is the default array for
160the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
a0d0e21e 161
0b9346e6 162See L<perlsub>.
a0d0e21e 163
1311257d 164=item $LIST_SEPARATOR
166=item $"
69520822 169When an array or an array slice is interpolated into a double-quoted
170string or a similar context such as C</.../>, its elements are
171separated by this value. Default is a space. For example, this:
0b9346e6 173 print "The array is: @array\n";
69520822 174
175is equivalent to this:
0b9346e6 177 print "The array is: " . join($", @array) . "\n";
69520822 178
179Mnemonic: works in double-quoted context.
1311257d 180
b0c22438 181=item $PROCESS_ID
cde0cee5 182
b0c22438 183=item $PID
a0d0e21e 184
b0c22438 185=item $$
186X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 187
b0c22438 188The process number of the Perl running this script. You should
189consider this variable read-only, although it will be altered
190across C<fork()> calls.
a0d0e21e 191
b0c22438 192Note for Linux users: on Linux, the C functions C<getpid()> and
193C<getppid()> return different values from different threads. In order to
194be portable, this behavior is not reflected by C<$$>, whose value remains
195consistent across threads. If you want to call the underlying C<getpid()>,
196you may use the CPAN module C<Linux::Pid>.
a0d0e21e 197
b0c22438 198Mnemonic: same as shells.
ad83b128 199
b0c22438 200=item $REAL_GROUP_ID
a01268b5 201
b0c22438 202=item $GID
a01268b5 203
b0c22438 204=item $(
205X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 206
b0c22438 207The real gid of this process. If you are on a machine that supports
208membership in multiple groups simultaneously, gives a space separated
209list of groups you are in. The first number is the one returned by
210C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
211the same as the first number.
a01268b5 212
b0c22438 213However, a value assigned to C<$(> must be a single number used to
214set the real gid. So the value given by C<$(> should I<not> be assigned
215back to C<$(> without being forced numeric, such as by adding zero. Note
216that this is different to the effective gid (C<$)>) which does take a
fe307981 218
b0c22438 219You can change both the real gid and the effective gid at the same
220time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
221to detect any possible errors after an attempted change.
6cef1e77 222
b0c22438 223Mnemonic: parentheses are used to I<group> things. The real gid is the
224group you I<left>, if you're running setgid.
6cef1e77 225
b0c22438 226=item $EFFECTIVE_GROUP_ID
8e08999f 227
b0c22438 228=item $EGID
81714fb9 229
b0c22438 230=item $)
81714fb9 232
b0c22438 233The effective gid of this process. If you are on a machine that
234supports membership in multiple groups simultaneously, gives a space
235separated list of groups you are in. The first number is the one
236returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
237one of which may be the same as the first number.
81714fb9 238
b0c22438 239Similarly, a value assigned to C<$)> must also be a space-separated
240list of numbers. The first number sets the effective gid, and
241the rest (if any) are passed to C<setgroups()>. To get the effect of an
242empty list for C<setgroups()>, just repeat the new effective gid; that is,
243to force an effective gid of 5 and an effectively empty C<setgroups()>
244list, say C< $) = "5 5" >.
81714fb9 245
b0c22438 246You can change both the effective gid and the real gid at the same
247time by using C<POSIX::setgid()> (use only a single numeric argument).
248Changes to C<$)> require a check to C<$!> to detect any possible errors
249after an attempted change.
44a2ac75 250
b0c22438 251C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
252machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
253and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 254
b0c22438 255Mnemonic: parentheses are used to I<group> things. The effective gid
256is the group that's I<right> for you, if you're running setgid.
44a2ac75 257
b0c22438 258=item $PROGRAM_NAME
a0d0e21e 259
b0c22438 260=item $0
a0d0e21e 262
b0c22438 263Contains the name of the program being executed.
a0d0e21e 264
69520822 265On some (but not all) operating systems assigning to C<$0> modifies
7333b1c4 266the argument area that the C<ps> program sees. On some platforms you
b0c22438 267may have to use special C<ps> options or a different C<ps> to see the
7333b1c4 268changes. Modifying the C<$0> is more useful as a way of indicating the
b0c22438 269current program state than it is for hiding the program you're
a0d0e21e 271
69520822 272Note that there are platform-specific limitations on the maximum
b0c22438 273length of C<$0>. In the most extreme case it may be limited to the
274space occupied by the original C<$0>.
fcc7d916 275
b0c22438 276In some platforms there may be arbitrary amount of padding, for
277example space characters, after the modified name as shown by C<ps>.
278In some platforms this padding may extend all the way to the original
279length of the argument area, no matter what you do (this is the case
280for example with Linux 2.2).
fcc7d916 281
b0c22438 282Note for BSD users: setting C<$0> does not completely remove "perl"
283from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
284result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
285and the " (perl)" suffix are shown depends on your exact BSD variant
286and version). This is an operating system feature, Perl cannot help it.
fcc7d916 287
b0c22438 288In multithreaded scripts Perl coordinates the threads so that any
289thread may modify its copy of the C<$0> and the change becomes visible
290to ps(1) (assuming the operating system plays along). Note that
291the view of C<$0> the other threads have will not change since they
292have their own copies of it.
fcc7d916 293
b0c22438 294If the program has been given to perl via the switches C<-e> or C<-E>,
295C<$0> will contain the string C<"-e">.
fcc7d916 296
b0c22438 297On Linux as of perl 5.14 the legacy process name will be set with
0b9346e6 298C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
b0c22438 299perl has done since version 4.000. Now system utilities that read the
300legacy process name such as ps, top and killall will recognize the
301name you set when assigning to C<$0>. The string you supply will be
302cut off at 16 bytes, this is a limitation imposed by Linux.
fcc7d916 303
b0c22438 304Mnemonic: same as B<sh> and B<ksh>.
0b9346e6 305
308=item $SUBSEP
310=item $;
313The subscript separator for multidimensional array emulation. If you
314refer to a hash element as
316 $foo{$a,$b,$c}
318it really means
320 $foo{join($;, $a, $b, $c)}
322But don't put
324 @foo{$a,$b,$c} # a slice--note the @
326which means
328 ($foo{$a},$foo{$b},$foo{$c})
7333b1c4 330Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 331binary data there might not be any safe value for C<$;>.
333Consider using "real" multidimensional arrays as described
334in L<perllol>.
336Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
338=item $REAL_USER_ID
340=item $UID
342=item $<
343X<< $< >> X<$UID> X<$REAL_USER_ID>
345The real uid of this process. You can change both the real uid and the
346effective uid at the same time by using C<POSIX::setuid()>. Since
347changes to C<< $< >> require a system call, check C<$!> after a change
348attempt to detect any possible errors.
350Mnemonic: it's the uid you came I<from>, if you're running setuid.
354=item $EUID
356=item $>
359The effective uid of this process. For example:
361 $< = $>; # set real to effective uid
362 ($<,$>) = ($>,$<); # swap real and effective uids
364You can change both the effective uid and the real uid at the same
365time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
366to C<$!> to detect any possible errors after an attempted change.
368C<< $< >> and C<< $> >> can be swapped only on machines
369supporting C<setreuid()>.
371Mnemonic: it's the uid you went I<to>, if you're running setuid.
373=item $a
375=item $b
376X<$a> X<$b>
378Special package variables when using C<sort()>, see L<perlfunc/sort>.
379Because of this specialness C<$a> and C<$b> don't need to be declared
380(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
381pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
382be able to use them in the C<sort()> comparison block or function.
b0c22438 384=item $COMPILING
a0d0e21e 385
b0c22438 386=item $^C
a0d0e21e 388
b0c22438 389The current value of the flag associated with the B<-c> switch.
390Mainly of use with B<-MO=...> to allow code to alter its behavior
391when being compiled, such as for example to C<AUTOLOAD> at compile
7333b1c4 392time rather than normal, deferred loading. Setting
b0c22438 393C<$^C = 1> is similar to calling C<B::minus_c>.
a0d0e21e 394
b0c22438 395This variable was added in Perl 5.6.
a0d0e21e 396
b0c22438 397=item $DEBUGGING
a0d0e21e 398
b0c22438 399=item $^D
a0d0e21e 401
b0c22438 402The current value of the debugging flags. May be read or set. Like its
403command-line equivalent, you can use numeric or symbolic values, eg
404C<$^D = 10> or C<$^D = "st">.
68dc0745 405
b0c22438 406Mnemonic: value of B<-D> switch.
5b2b9c68 407
0b9346e6 408=item ${^ENCODING}
5b442a2a 409X<${^ENCODING}>
0b9346e6 410
411The I<object reference> to the C<Encode> object that is used to convert
412the source code to Unicode. Thanks to this variable your Perl script
413does not have to be written in UTF-8. Default is I<undef>. The direct
414manipulation of this variable is highly discouraged.
416This variable was added in Perl 5.8.2.
418=item %ENV
421The hash C<%ENV> contains your current environment. Setting a
422value in C<ENV> changes the environment for any child processes
423you subsequently C<fork()> off.
b0c22438 425=item $SYSTEM_FD_MAX
5b2b9c68 426
b0c22438 427=item $^F
5b2b9c68 429
b0c22438 430The maximum system file descriptor, ordinarily 2. System file
431descriptors are passed to C<exec()>ed processes, while higher file
432descriptors are not. Also, during an C<open()>, system file descriptors are
433preserved even if the C<open()> fails (ordinary file descriptors are
434closed before the C<open()> is attempted). The close-on-exec
435status of a file descriptor will be decided according to the value of
436C<$^F> when the corresponding file, pipe, or socket was opened, not the
437time of the C<exec()>.
5b2b9c68 438
0b9346e6 439=item @F
442The array C<@F> contains the fields of each line read in when autosplit
7333b1c4 443mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 444is package-specific, and must be declared or given a full package name
445if not in package main when running under C<strict 'vars'>.
10c97e5d 447=item ${^GLOBAL_PHASE}
449The current phase of the perl interpreter.
451Possible values include:
453=over 8
455=item CONSTRUCT
457The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
458value is mostly there for completeness and for use via the
459underlying C variable C<PL_phase>. It's not really possible for Perl
460code to be executed unless construction of the interpreter is
463=item START
465This is the global compile-time. That includes, basically, every
466C<BEGIN> block executed directly or indirectly from during the
467compile-time of the top-level program.
469This phase is not called "BEGIN" to avoid confusion with
470C<BEGIN>-blocks, as those are executed during compile-time of any
471compilation unit, not just the top-level program. A new, localised
472compile-time entered at run-time, for example by constructs as
473C<eval "use SomeModule"> are not global interpreter phases, and
474therefore aren't reflected by C<${^GLOBAL_PHASE}>.
476=item CHECK
478Execution of any C<CHECK> blocks.
480=item INIT
482Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
484=item RUN
486The main run-time, i.e. the execution of C<PL_main_root>.
488=item END
490Execution of any C<END> blocks.
492=item DESTRUCT
494Global destruction.
498Also note that there's no value for UNITCHECK-blocks. That's because
499those are run for each compilation unit individually, and therefore is
500not a global interpreter phase.
502Not every program has to go through each of the possible phases, but
503transition from one phase to another can only happen in the order
504described in the above list.
506The patch also includes some basic tests, if you prefer actual working
507examples of how C<${^GLOBAL_PHASE}> behaves.
509This variable was added in Perl 5.13.7.
b0c22438 511=item $^H
5b442a2a 512X<$^H>
883faa13 513
b0c22438 514WARNING: This variable is strictly for internal use only. Its availability,
515behavior, and contents are subject to change without notice.
a0d0e21e 516
b0c22438 517This variable contains compile-time hints for the Perl interpreter. At the
518end of compilation of a BLOCK the value of this variable is restored to the
519value when the interpreter started to compile the BLOCK.
a0d0e21e 520
b0c22438 521When perl begins to parse any block construct that provides a lexical scope
522(e.g., eval body, required file, subroutine body, loop body, or conditional
523block), the existing value of C<$^H> is saved, but its value is left unchanged.
524When the compilation of the block is completed, it regains the saved value.
525Between the points where its value is saved and restored, code that
526executes within BEGIN blocks is free to change the value of C<$^H>.
a0d0e21e 527
b0c22438 528This behavior provides the semantic of lexical scoping, and is used in,
529for instance, the C<use strict> pragma.
a0d0e21e 530
b0c22438 531The contents should be an integer; different bits of it are used for
532different pragmatic flags. Here's an example:
a0d0e21e 533
0b9346e6 534 sub add_100 { $^H |= 0x100 }
a0d0e21e 535
0b9346e6 536 sub foo {
537 BEGIN { add_100() }
538 bar->baz($boon);
539 }
a0d0e21e 540
b0c22438 541Consider what happens during execution of the BEGIN block. At this point
542the BEGIN block has already been compiled, but the body of C<foo()> is still
543being compiled. The new value of C<$^H> will therefore be visible only while
544the body of C<foo()> is being compiled.
a0d0e21e 545
7333b1c4 546Substitution of C<BEGIN { add_100() }> block with:
a0d0e21e 547
0b9346e6 548 BEGIN { require strict; strict->import('vars') }
a0d0e21e 549
7333b1c4 550demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
b0c22438 551version of the same lexical pragma:
a0d0e21e 552
0b9346e6 553 BEGIN { require strict; strict->import('vars') if $condition }
a0d0e21e 554
b0c22438 555This variable was added in Perl 5.003.
a0d0e21e 556
b0c22438 557=item %^H
5b442a2a 558X<%^H>
a0d0e21e 559
b0c22438 560The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
561useful for implementation of lexically scoped pragmas. See L<perlpragma>.
a0d0e21e 562
b0c22438 563This variable was added in Perl 5.6.
a0d0e21e 564
0b9346e6 565=item @INC
568The array C<@INC> contains the list of places that the C<do EXPR>,
7333b1c4 569C<require>, or C<use> constructs look for their library files. It
0b9346e6 570initially consists of the arguments to any B<-I> command-line
571switches, followed by the default Perl library, probably
572F</usr/local/lib/perl>, followed by ".", to represent the current
7333b1c4 573directory. ("." will not be appended if taint checks are enabled,
0b9346e6 574either by C<-T> or by C<-t>.) If you need to modify this at runtime,
575you should use the C<use lib> pragma to get the machine-dependent
576library properly loaded also:
578 use lib '/mypath/libdir/';
579 use SomeMod;
581You can also insert hooks into the file inclusion system by putting Perl
582code directly into C<@INC>. Those hooks may be subroutine references, array
583references or blessed objects. See L<perlfunc/require> for details.
585=item %INC
588The hash C<%INC> contains entries for each filename included via the
589C<do>, C<require>, or C<use> operators. The key is the filename
590you specified (with module names converted to pathnames), and the
591value is the location of the file found. The C<require>
592operator uses this hash to determine whether a particular file has
593already been included.
595If the file was loaded via a hook (e.g. a subroutine reference, see
596L<perlfunc/require> for a description of these hooks), this hook is
597by default inserted into C<%INC> in place of a filename. Note, however,
598that the hook may have set the C<%INC> entry by itself to provide some more
599specific info.
b0c22438 601=item $INPLACE_EDIT
a0d0e21e 602
b0c22438 603=item $^I
a0d0e21e 605
b0c22438 606The current value of the inplace-edit extension. Use C<undef> to disable
607inplace editing.
a0d0e21e 608
b0c22438 609Mnemonic: value of B<-i> switch.
a0d0e21e 610
b0c22438 611=item $^M
a0d0e21e 613
b0c22438 614By default, running out of memory is an untrappable, fatal error.
615However, if suitably built, Perl can use the contents of C<$^M>
616as an emergency memory pool after C<die()>ing. Suppose that your Perl
617were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
a0d0e21e 619
0b9346e6 620 $^M = 'a' x (1 << 16);
a0d0e21e 621
b0c22438 622would allocate a 64K buffer for use in an emergency. See the
623F<INSTALL> file in the Perl distribution for information on how to
624add custom C compilation flags when compiling perl. To discourage casual
625use of this advanced feature, there is no L<English|English> long name for
626this variable.
a0d0e21e 627
b0c22438 628This variable was added in Perl 5.004.
a0d0e21e 629
b0c22438 630=item $OSNAME
a0d0e21e 631
b0c22438 632=item $^O
633X<$^O> X<$OSNAME>
a0d0e21e 634
b0c22438 635The name of the operating system under which this copy of Perl was
636built, as determined during the configuration process. For examples
637see L<perlport/PLATFORMS>.
a0d0e21e 638
b0c22438 639The value is identical to C<$Config{'osname'}>. See also L<Config>
640and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 641
b0c22438 642In Windows platforms, C<$^O> is not very helpful: since it is always
643C<MSWin32>, it doesn't tell the difference between
64495/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
645Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
646between the variants.
a0d0e21e 647
b0c22438 648This variable was added in Perl 5.003.
a0d0e21e 649
b0c22438 650=item ${^OPEN}
5b442a2a 651X<${^OPEN}>
a0d0e21e 652
b0c22438 653An internal variable used by PerlIO. A string in two parts, separated
654by a C<\0> byte, the first part describes the input layers, the second
655part describes the output layers.
a0d0e21e 656
b0c22438 657This variable was added in Perl 5.8.2.
a0d0e21e 658
b0c22438 659=item $PERLDB
a0d0e21e 660
b0c22438 661=item $^P
662X<$^P> X<$PERLDB>
a0d0e21e 663
b0c22438 664The internal variable for debugging support. The meanings of the
665various bits are subject to change, but currently indicate:
a0d0e21e 666
b0c22438 667=over 6
a0d0e21e 668
b0c22438 669=item 0x01
a0d0e21e 670
b0c22438 671Debug subroutine enter/exit.
a0d0e21e 672
b0c22438 673=item 0x02
a0d0e21e 674
b0c22438 675Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
676statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 677
b0c22438 678=item 0x04
fe307981 679
b0c22438 680Switch off optimizations.
6cef1e77 681
b0c22438 682=item 0x08
6cef1e77 683
b0c22438 684Preserve more data for future interactive inspections.
6cef1e77 685
b0c22438 686=item 0x10
4ba05bdc 687
b0c22438 688Keep info about source lines on which a subroutine is defined.
4ba05bdc 689
b0c22438 690=item 0x20
4ba05bdc 691
b0c22438 692Start with single-step on.
4ba05bdc 693
b0c22438 694=item 0x40
4ba05bdc 695
b0c22438 696Use subroutine address instead of name when reporting.
4ba05bdc 697
b0c22438 698=item 0x80
4ba05bdc 699
b0c22438 700Report C<goto &subroutine> as well.
4ba05bdc 701
b0c22438 702=item 0x100
4ba05bdc 703
b0c22438 704Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 705
b0c22438 706=item 0x200
44a2ac75 707
b0c22438 708Provide informative names to anonymous subroutines based on the place they
709were compiled.
44a2ac75 710
b0c22438 711=item 0x400
44a2ac75 712
b0c22438 713Save source code lines into C<@{"_<$filename"}>.
44a2ac75 714
b0c22438 715=back
44a2ac75 716
b0c22438 717Some bits may be relevant at compile-time only, some at
7333b1c4 718run-time only. This is a new mechanism and the details may change.
b0c22438 719See also L<perldebguts>.
3195cf34 720
b0c22438 721=item %SIG
b0c22438 722X<%SIG>
a0d0e21e 723
b0c22438 724The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 725
0b9346e6 726 sub handler { # 1st argument is signal name
727 my($sig) = @_;
728 print "Caught a SIG$sig--shutting down\n";
729 close(LOG);
730 exit(0);
731 }
a0d0e21e 732
0b9346e6 733 $SIG{'INT'} = \&handler;
734 $SIG{'QUIT'} = \&handler;
735 ...
736 $SIG{'INT'} = 'DEFAULT'; # restore default action
737 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 738
b0c22438 739Using a value of C<'IGNORE'> usually has the effect of ignoring the
740signal, except for the C<CHLD> signal. See L<perlipc> for more about
741this special case.
a0d0e21e 742
b0c22438 743Here are some other examples:
a0d0e21e 744
0b9346e6 745 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
746 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
747 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
748 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 749
b0c22438 750Be sure not to use a bareword as the name of a signal handler,
751lest you inadvertently call it.
a0d0e21e 752
b0c22438 753If your system has the C<sigaction()> function then signal handlers
754are installed using it. This means you get reliable signal handling.
7b8d334a 755
b0c22438 756The default delivery policy of signals changed in Perl 5.8.0 from
757immediate (also known as "unsafe") to deferred, also known as "safe
7333b1c4 758signals". See L<perlipc> for more information.
aa689395 759
b0c22438 760Certain internal hooks can be also set using the C<%SIG> hash. The
761routine indicated by C<$SIG{__WARN__}> is called when a warning
7333b1c4 762message is about to be printed. The warning message is passed as the
763first argument. The presence of a C<__WARN__> hook causes the
b0c22438 764ordinary printing of warnings to C<STDERR> to be suppressed. You can
765use this to save warnings in a variable, or turn warnings into fatal
766errors, like this:
19799a22 767
0b9346e6 768 local $SIG{__WARN__} = sub { die $_[0] };
769 eval $proggie;
a8f8344d 770
b0c22438 771As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
772disable warnings using the empty subroutine:
f86702cc 773
0b9346e6 774 local $SIG{__WARN__} = sub {};
55602bd2 775
b0c22438 776The routine indicated by C<$SIG{__DIE__}> is called when a fatal
777exception is about to be thrown. The error message is passed as the
778first argument. When a C<__DIE__> hook routine returns, the exception
779processing continues as it would have in the absence of the hook,
780unless the hook routine itself exits via a C<goto>, a loop exit, or a
781C<die()>. The C<__DIE__> handler is explicitly disabled during the
782call, so that you can die from a C<__DIE__> handler. Similarly for
e5218da5 784
b0c22438 785Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
786even inside an C<eval()>. Do not use this to rewrite a pending
787exception in C<$@>, or as a bizarre substitute for overriding
788C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
789in a future release so that C<$SIG{__DIE__}> is only called if your
790program is about to exit, as was the original intent. Any other use is
793C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
794may be called to report (probable) errors found by the parser. In such
795a case the parser may be in inconsistent state, so any attempt to
796evaluate Perl code from such a handler will probably result in a
797segfault. This means that warnings or errors that result from parsing
798Perl should be used with extreme caution, like this:
e5218da5 799
0b9346e6 800 require Carp if defined $^S;
801 Carp::confess("Something wrong") if defined &Carp::confess;
802 die "Something wrong, but could not load Carp to give backtrace...
803 To see backtrace try starting Perl with -MCarp switch";
e5218da5 804
b0c22438 805Here the first line will load C<Carp> I<unless> it is the parser who
806called the handler. The second line will print backtrace and die if
807C<Carp> was available. The third line will be executed only if C<Carp> was
808not available.
0a378802 809
0b9346e6 810Having to even think about the C<$^S> variable in your exception
7333b1c4 811handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
0b9346e6 812invites grievous and difficult to track down errors. Avoid it
813and use an C<END{}> or CORE::GLOBAL::die override instead.
b0c22438 815See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
816L<warnings> for additional information.
0a378802 817
b0c22438 818=item $BASETIME
6ab308ee 819
b0c22438 820=item $^T
821X<$^T> X<$BASETIME>
6ab308ee 822
b0c22438 823The time at which the program began running, in seconds since the
824epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
825and B<-C> filetests are based on this value.
a0d0e21e 826
b0c22438 827=item ${^TAINT}
5b442a2a 828X<${^TAINT}>
55602bd2 829
b0c22438 830Reflects if taint mode is on or off. 1 for on (the program was run with
831B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 832B<-t> or B<-TU>).
daaddde1 833
b0c22438 834This variable is read-only.
daaddde1 835
b0c22438 836This variable was added in Perl 5.8.
4c5cef9b 837
b0c22438 838=item ${^UNICODE}
5b442a2a 839X<${^UNICODE}>
4c5cef9b 840
7333b1c4 841Reflects certain Unicode settings of Perl. See L<perlrun>
b0c22438 842documentation for the C<-C> switch for more information about
0b9346e6 843the possible values.
5c055ba3 844
b0c22438 845This variable is set during Perl startup and is thereafter read-only.
5c055ba3 846
b0c22438 847This variable was added in Perl 5.8.2.
22fae026 848
b0c22438 849=item ${^UTF8CACHE}
5b442a2a 850X<${^UTF8CACHE}>
22fae026 851
b0c22438 852This variable controls the state of the internal UTF-8 offset caching code.
8531 for on (the default), 0 for off, -1 to debug the caching code by checking
854all its results against linear scans, and panicking on any discrepancy.
22fae026 855
b0c22438 856This variable was added in Perl 5.8.9.
22fae026 857
b0c22438 858=item ${^UTF8LOCALE}
5b442a2a 859X<${^UTF8LOCALE}>
5c055ba3 860
b0c22438 861This variable indicates whether a UTF-8 locale was detected by perl at
862startup. This information is used by perl when it's in
863adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
864switch); see L<perlrun> for more info on this.
55602bd2 865
b0c22438 866This variable was added in Perl 5.8.8.
a0d0e21e 867
b0c22438 868=item $PERL_VERSION
a0d0e21e 869
b0c22438 870=item $^V
a0d0e21e 872
b0c22438 873The revision, version, and subversion of the Perl interpreter,
874represented as a C<version> object.
748a9306 875
b0c22438 876This variable first appeared in perl 5.6.0; earlier versions of perl
877will see an undefined value. Before perl 5.10.0 C<$^V> was represented
878as a v-string.
55602bd2 879
b0c22438 880C<$^V> can be used to determine whether the Perl interpreter executing
881a script is in the right range of versions. For example:
a0d0e21e 882
0b9346e6 883 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 884
b0c22438 885To convert C<$^V> into its string representation use C<sprintf()>'s
886C<"%vd"> conversion:
a0d0e21e 887
0b9346e6 888 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 889
b0c22438 890See the documentation of C<use VERSION> and C<require VERSION>
891for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 892
b0c22438 893See also C<$]> for an older representation of the Perl version.
a0d0e21e 894
b0c22438 895This variable was added in Perl 5.6.
a0d0e21e 896
b0c22438 897Mnemonic: use ^V for Version Control.
a0d0e21e 898
b0c22438 899=item ${^WIN32_SLOPPY_STAT}
5b442a2a 900X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<>
a0d0e21e 901
b0c22438 902If this variable is set to a true value, then C<stat()> on Windows will
903not try to open the file. This means that the link count cannot be
904determined and file attributes may be out of date if additional
905hardlinks to the file exist. On the other hand, not opening the file
906is considerably faster, especially for files on network drives.
a0d0e21e 907
b0c22438 908This variable could be set in the F<> file to
909configure the local Perl installation to use "sloppy" C<stat()> by
910default. See the documentation for B<-f> in
911L<perlrun|perlrun/"Command Switches"> for more information about site
a0d0e21e 913
b0c22438 914This variable was added in Perl 5.10.
a0d0e21e 915
b0c22438 916=item $EXECUTABLE_NAME
a0d0e21e 917
b0c22438 918=item $^X
a0d0e21e 920
b0c22438 921The name used to execute the current copy of Perl, from C's
922C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 923
b0c22438 924Depending on the host operating system, the value of C<$^X> may be
925a relative or absolute pathname of the perl program file, or may
926be the string used to invoke perl but not the pathname of the
927perl program file. Also, most operating systems permit invoking
928programs that are not in the PATH environment variable, so there
929is no guarantee that the value of C<$^X> is in PATH. For VMS, the
930value may or may not include a version number.
a0d0e21e 931
b0c22438 932You usually can use the value of C<$^X> to re-invoke an independent
933copy of the same perl that is currently running, e.g.,
a0d0e21e 934
0b9346e6 935 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 936
b0c22438 937But recall that not all operating systems support forking or
938capturing of the output of commands, so this complex statement
939may not be portable.
a0d0e21e 940
b0c22438 941It is not safe to use the value of C<$^X> as a path name of a file,
942as some operating systems that have a mandatory suffix on
943executable files do not require use of the suffix when invoking
944a command. To convert the value of C<$^X> to a path name, use the
945following statements:
8cc95fdb 946
0b9346e6 947 # Build up a set of file names (not command names).
948 use Config;
949 my $this_perl = $^X;
950 if ($^O ne 'VMS') {
951 $this_perl .= $Config{_exe}
952 unless $this_perl =~ m/$Config{_exe}$/i;
953 }
8cc95fdb 954
b0c22438 955Because many operating systems permit anyone with read access to
956the Perl program file to make a copy of it, patch the copy, and
957then execute the copy, the security-conscious Perl programmer
958should take care to invoke the installed copy of perl, not the
959copy referenced by C<$^X>. The following statements accomplish
960this goal, and produce a pathname that can be invoked as a
961command or referenced as a file.
a043a685 962
0b9346e6 963 use Config;
964 my $secure_perl_path = $Config{perlpath};
965 if ($^O ne 'VMS') {
966 $secure_perl_path .= $Config{_exe}
967 unless $secure_perl_path =~ m/$Config{_exe}$/i;
968 }
a0d0e21e 969
b0c22438 970=back
a0d0e21e 971
b0c22438 972=head2 Variables related to regular expressions
974Most of the special variables related to regular expressions are side
975effects. Perl sets these variables when it has a successful match, so
976you should check the match result before using them. For instance:
978 if( /P(A)TT(ER)N/ ) {
979 print "I found $1 and $2\n";
980 }
0b9346e6 982These variables are read-only and dynamically-scoped, unless we note
b0c22438 983otherwise.
0b9346e6 985The dynamic nature of the regular expression variables means that
986their value is limited to the block that they are in, as demonstrated
987by this bit of code:
b0c22438 988
989 my $outer = 'Wallace and Grommit';
990 my $inner = 'Mutt and Jeff';
0b9346e6 991
b0c22438 992 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 993
b0c22438 994 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 995
b0c22438 996 {
997 OUTER:
998 show_n() if $outer =~ m/$pattern/;
0b9346e6 999
b0c22438 1000 INNER: {
1001 show_n() if $inner =~ m/$pattern/;
1002 }
0b9346e6 1003
b0c22438 1004 show_n();
1005 }
0b9346e6 1007The output shows that while in the C<OUTER> block, the values of C<$1>
1008and C<$2> are from the match against C<$outer>. Inside the C<INNER>
1009block, the values of C<$1> and C<$2> are from the match against
1010C<$inner>, but only until the end of the block (i.e. the dynamic
1011scope). After the C<INNER> block completes, the values of C<$1> and
1012C<$2> return to the values for the match against C<$outer> even though
b0c22438 1013we have not made another match:
1015 $1 is Wallace; $2 is Grommit
1016 $1 is Mutt; $2 is Jeff
1017 $1 is Wallace; $2 is Grommit
a0d0e21e 1018
0b9346e6 1019Due to an unfortunate accident of Perl's implementation, C<use
1020English> imposes a considerable performance penalty on all regular
1021expression matches in a program because it uses the C<$`>, C<$&>, and
1022C<$'>, regardless of whether they occur in the scope of C<use
1023English>. For that reason, saying C<use English> in libraries is
1024strongly discouraged unless you import it without the match variables:
1026 use English '-no_match_vars'
1028The C<Devel::NYTProf> module can help you find uses of these
1029problematic match variables in your code.
1031Since Perl 5.10, you can use the C</p> match operator flag and the
1032C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
1033so you only suffer the performance penalties.
b0c22438 1035=over 8
a0d0e21e 1036
b0c22438 1037=item $<I<digits>> ($1, $2, ...)
1038X<$1> X<$2> X<$3>
8cc95fdb 1039
b0c22438 1040Contains the subpattern from the corresponding set of capturing
1041parentheses from the last successful pattern match, not counting patterns
1042matched in nested blocks that have been exited already.
8cc95fdb 1043
b0c22438 1044These variables are read-only and dynamically-scoped.
a043a685 1045
b0c22438 1046Mnemonic: like \digits.
a0d0e21e 1047
b0c22438 1048=item $MATCH
a0d0e21e 1049
b0c22438 1050=item $&
1051X<$&> X<$MATCH>
a0d0e21e 1052
b0c22438 1053The string matched by the last successful pattern match (not counting
1054any matches hidden within a BLOCK or C<eval()> enclosed by the current
a0d0e21e 1056
b0c22438 1057The use of this variable anywhere in a program imposes a considerable
0b9346e6 1058performance penalty on all regular expression matches. To avoid this
1059penalty, you can extract the same substring by using L</@->. Starting
1060with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
1061variable to do the same thing for particular match operations.
80bca1b4 1062
b0c22438 1063This variable is read-only and dynamically-scoped.
f9cbb277 1064
b0c22438 1065Mnemonic: like C<&> in some editors.
0b9346e6 1066
b0c22438 1067=item ${^MATCH}
a0d0e21e 1069
b0c22438 1070This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1071performance penalty associated with that variable, and is only guaranteed
1072to return a defined value when the pattern was compiled or executed with
1073the C</p> modifier.
80bca1b4 1074
b0c22438 1075This variable was added in Perl 5.10.
4bc88a62 1076
b0c22438 1077This variable is read-only and dynamically-scoped.
e2975953 1078
b0c22438 1079=item $PREMATCH
52c447a8 1080
b0c22438 1081=item $`
5b442a2a 1082X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 1083
b0c22438 1084The string preceding whatever was matched by the last successful
1085pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1086enclosed by the current BLOCK.
a0d0e21e 1087
b0c22438 1088The use of this variable anywhere in a program imposes a considerable
0b9346e6 1089performance penalty on all regular expression matches. To avoid this
1090penalty, you can extract the same substring by using L</@->. Starting
1091with Perl 5.10, you can use the </p> match flag and the
1092C<${^PREMATCH}> variable to do the same thing for particular match
a0d0e21e 1094
b0c22438 1095This variable is read-only and dynamically-scoped.
a0d0e21e 1096
b0c22438 1097Mnemonic: C<`> often precedes a quoted string.
f83ed198 1098
b0c22438 1099=item ${^PREMATCH}
5b442a2a 1100X<$`> X<${^PREMATCH}>
a0d0e21e 1101
b0c22438 1102This is similar to C<$`> ($PREMATCH) except that it does not incur the
1103performance penalty associated with that variable, and is only guaranteed
1104to return a defined value when the pattern was compiled or executed with
1105the C</p> modifier.
a0d0e21e 1106
b0c22438 1107This variable was added in Perl 5.10
a0d0e21e 1108
b0c22438 1109This variable is read-only and dynamically-scoped.
a0d0e21e 1110
b0c22438 1111=item $POSTMATCH
16070b82 1112
b0c22438 1113=item $'
5b442a2a 1114X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1115
b0c22438 1116The string following whatever was matched by the last successful
1117pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1118enclosed by the current BLOCK). Example:
305aace0 1119
0b9346e6 1120 local $_ = 'abcdefghi';
1121 /def/;
1122 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1123
b0c22438 1124The use of this variable anywhere in a program imposes a considerable
0b9346e6 1125performance penalty on all regular expression matches.
1126To avoid this penalty, you can extract the same substring by
b0c22438 1127using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1128and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1129match operations.
a0d0e21e 1130
b0c22438 1131This variable is read-only and dynamically-scoped.
1133Mnemonic: C<'> often follows a quoted string.
1135=item ${^POSTMATCH}
5b442a2a 1136X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1137
1138This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1139performance penalty associated with that variable, and is only guaranteed
1140to return a defined value when the pattern was compiled or executed with
1141the C</p> modifier.
1143This variable was added in Perl 5.10.
1145This variable is read-only and dynamically-scoped.
1149=item $+
1152The text matched by the last bracket of the last successful search pattern.
1153This is useful if you don't know which one of a set of alternative patterns
1154matched. For example:
0b9346e6 1156 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1157
1158This variable is read-only and dynamically-scoped.
1160Mnemonic: be positive and forward looking.
1164=item $^N
5b442a2a 1165X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1166
1167The text matched by the used group most-recently closed (i.e. the group
1168with the rightmost closing parenthesis) of the last successful search
1171This is primarily used inside C<(?{...})> blocks for examining text
1172recently matched. For example, to effectively capture text to a variable
1173(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
0b9346e6 1175 (?:(...)(?{ $var = $^N }))
b0c22438 1176
1177By setting and then using C<$var> in this way relieves you from having to
1178worry about exactly which numbered set of parentheses they are.
1180This variable was added in Perl 5.8.
1182Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1184=item @LAST_MATCH_END
1186=item @+
1187X<@+> X<@LAST_MATCH_END>
1189This array holds the offsets of the ends of the last successful
1190submatches in the currently active dynamic scope. C<$+[0]> is
1191the offset into the string of the end of the entire match. This
1192is the same value as what the C<pos> function returns when called
1193on the variable that was matched against. The I<n>th element
1194of this array holds the offset of the I<n>th submatch, so
1195C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
7333b1c4 1196past where C<$2> ends, and so on. You can use C<$#+> to determine
b0c22438 1197how many subgroups were in the last successful match. See the
1198examples given for the C<@-> variable.
1200This variable was added in Perl 5.6.
1204=item %+
5b442a2a 1205X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1206
1207Similar to C<@+>, the C<%+> hash allows access to the named capture
1208buffers, should they exist, in the last successful match in the
1209currently active dynamic scope.
1211For example, C<$+{foo}> is equivalent to C<$1> after the following match:
0b9346e6 1213 'foo' =~ /(?<foo>foo)/;
b0c22438 1214
1215The keys of the C<%+> hash list only the names of buffers that have
1216captured (and that are thus associated to defined values).
1218The underlying behaviour of C<%+> is provided by the
1219L<Tie::Hash::NamedCapture> module.
1221B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1222associated with the last successful regular expression. Therefore mixing
1223iterative access to them via C<each> may have unpredictable results.
1224Likewise, if the last successful match changes, then the results may be
1227This variable was added in Perl 5.10.
a0d0e21e 1228
b0c22438 1229This variable is read-only and dynamically-scoped.
1233=item @-
1236C<$-[0]> is the offset of the start of the last successful match.
1237C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1238I<n>-th subpattern, or undef if the subpattern did not match.
1240Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1241$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1242$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1243C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1244matched subgroup in the last successful match. Contrast with
1245C<$#+>, the number of subgroups in the regular expression. Compare
1246with C<@+>.
1248This array holds the offsets of the beginnings of the last
1249successful submatches in the currently active dynamic scope.
1250C<$-[0]> is the offset into the string of the beginning of the
7333b1c4 1251entire match. The I<n>th element of this array holds the offset
b0c22438 1252of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1253begins, C<$-[2]> the offset where C<$2> begins, and so on.
1255After a match against some variable C<$var>:
1257=over 5
1259=item C<$`> is the same as C<substr($var, 0, $-[0])>
1261=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1263=item C<$'> is the same as C<substr($var, $+[0])>
1265=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1267=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1269=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1273This variable was added in Perl 5.6.
5b442a2a 1275=item %LAST_MATCH_START
b0c22438 1277=item %-
5b442a2a 1278X<%-> X<%LAST_MATCH_START>
b0c22438 1279
1280Similar to C<%+>, this variable allows access to the named capture groups
1281in the last successful match in the currently active dynamic scope. To
1282each capture group name found in the regular expression, it associates a
1283reference to an array containing the list of values captured by all
1284buffers with that name (should there be several of them), in the order
1285where they appear.
1287Here's an example:
1289 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1290 foreach my $bufname (sort keys %-) {
1291 my $ary = $-{$bufname};
1292 foreach my $idx (0..$#$ary) {
1293 print "\$-{$bufname}[$idx] : ",
1294 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1295 "\n";
1296 }
1297 }
1298 }
1300would print out:
0b9346e6 1302 $-{A}[0] : '1'
1303 $-{A}[1] : '3'
1304 $-{B}[0] : '2'
1305 $-{B}[1] : '4'
b0c22438 1306
1307The keys of the C<%-> hash correspond to all buffer names found in
1308the regular expression.
1310The behaviour of C<%-> is implemented via the
1311L<Tie::Hash::NamedCapture> module.
1313B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1314associated with the last successful regular expression. Therefore mixing
1315iterative access to them via C<each> may have unpredictable results.
1316Likewise, if the last successful match changes, then the results may be
1319This variable was added in Perl 5.10
1321This variable is read-only and dynamically-scoped.
1325=item $^R
1328The result of evaluation of the last successful C<(?{ code })>
1329regular expression assertion (see L<perlre>). May be written to.
1331This variable was added in Perl 5.005.
a0d0e21e 1332
a3621e74 1333=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1334X<${^RE_DEBUG_FLAGS}>
1336The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1337even when the C<re 'debug'> module is loaded. See L<re> for details.
1339This variable was added in Perl 5.10.
a3621e74 1340
0111c4fd 1341=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1342X<${^RE_TRIE_MAXBUF}>
1344Controls how certain regex optimisations are applied and how much memory they
1345utilize. This value by default is 65536 which corresponds to a 512kB temporary
1346cache. Set this to a higher value to trade memory for speed when matching
1347large alternations. Set it to a lower value if you want the optimisations to
1348be as conservative of memory as possible but still occur, and set it to a
1349negative value to prevent the optimisation and conserve the most memory.
1350Under normal situations this variable should be of no interest to you.
b0c22438 1352This variable was added in Perl 5.10.
a0d0e21e 1353
b0c22438 1354=back
a0d0e21e 1355
b0c22438 1356=head2 Variables related to filehandles
a0d0e21e 1357
b0c22438 1358Variables that depend on the currently selected filehandle may be set
1359by calling an appropriate object method on the C<IO::Handle> object,
1360although this is less efficient than using the regular built-in
1361variables. (Summary lines below for this contain the word HANDLE.)
1362First you must say
6e2995f4 1363
0b9346e6 1364 use IO::Handle;
0462a1ab 1365
b0c22438 1366after which you may use either
0462a1ab 1367
0b9346e6 1368 method HANDLE EXPR
0462a1ab 1369
b0c22438 1370or more safely,
0462a1ab 1371
0b9346e6 1372 HANDLE->method(EXPR)
0462a1ab 1373
b0c22438 1374Each method returns the old value of the C<IO::Handle> attribute. The
1375methods each take an optional EXPR, which, if supplied, specifies the
1376new value for the C<IO::Handle> attribute in question. If not
1377supplied, most methods do nothing to the current value--except for
1378C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1379
b0c22438 1380Because loading in the C<IO::Handle> class is an expensive operation,
1381you should learn how to use the regular built-in variables.
1383A few of these variables are considered "read-only". This means that
1384if you try to assign to this variable, either directly or indirectly
1385through a reference, you'll raise a run-time exception.
1387You should be very careful when modifying the default values of most
1388special variables described in this document. In most cases you want
1389to localize these variables before changing them, since if you don't,
1390the change may affect other modules which rely on the default values
1391of the special variables that you have changed. This is one of the
1392correct ways to read the whole file at once:
0b9346e6 1394 open my $fh, "<", "foo" or die $!;
1395 local $/; # enable localized slurp mode
1396 my $content = <$fh>;
1397 close $fh;
b0c22438 1398
1399But the following code is quite bad:
0b9346e6 1401 open my $fh, "<", "foo" or die $!;
1402 undef $/; # enable slurp mode
1403 my $content = <$fh>;
1404 close $fh;
b0c22438 1405
1406since some other module, may want to read data from some file in the
1407default "line mode", so if the code we have just presented has been
1408executed, the global value of C<$/> is now changed for any other code
1409running inside the same Perl interpreter.
1411Usually when a variable is localized you want to make sure that this
1412change affects the shortest scope possible. So unless you are already
1413inside some short C<{}> block, you should create one yourself. For
0b9346e6 1416 my $content = '';
1417 open my $fh, "<", "foo" or die $!;
1418 {
1419 local $/;
1420 $content = <$fh>;
1421 }
1422 close $fh;
0462a1ab 1423
b0c22438 1424Here is an example of how your own code can go broken:
0462a1ab 1425
0b9346e6 1426 for ( 1..3 ){
1427 $\ = "\r\n";
1428 nasty_break();
1429 print "$_";
1430 }
1432 sub nasty_break {
1433 $\ = "\f";
1434 # do something with $_
1435 }
0462a1ab 1436
0b9346e6 1437You probably expect this code to print the equivalent of
0462a1ab 1438
0b9346e6 1439 "1\r\n2\r\n3\r\n"
0462a1ab 1440
b0c22438 1441but instead you get:
0462a1ab 1442
0b9346e6 1443 "1\f2\f3\f"
0462a1ab 1444
0b9346e6 1445Why? Because C<nasty_break()> modifies C<$\> without localizing it
1446first. The value you set in C<nasty_break()> is still there when you
1447return. The fix is to add C<local()> so the value doesn't leak out of
6e2995f4 1449
0b9346e6 1450 local $\ = "\f";
a0d0e21e 1451
b0c22438 1452It's easy to notice the problem in such a short example, but in more
1453complicated code you are looking for trouble if you don't localize
1454changes to the special variables.
a0d0e21e 1455
b0c22438 1456=over 8
a0d0e21e 1457
b0c22438 1458=item $ARGV
fb73857a 1460
ca1b95ae 1461Contains the name of the current file when reading from C<< <> >>.
b0c22438 1462
1463=item @ARGV
ca1b95ae 1466The array C<@ARGV> contains the command-line arguments intended for
b0c22438 1467the script. C<$#ARGV> is generally the number of arguments minus
1468one, because C<$ARGV[0]> is the first argument, I<not> the program's
1469command name itself. See C<$0> for the command name.
84dabc03 1471=item ARGV
1474The special filehandle that iterates over command-line filenames in
1475C<@ARGV>. Usually written as the null filehandle in the angle operator
1476C<< <> >>. Note that currently C<ARGV> only has its magical effect
1477within the C<< <> >> operator; elsewhere it is just a plain filehandle
1478corresponding to the last file opened by C<< <> >>. In particular,
1479passing C<\*ARGV> as a parameter to a function that expects a filehandle
1480may not cause your function to automatically read the contents of all the
1481files in C<@ARGV>.
b0c22438 1483=item ARGVOUT
1486The special filehandle that points to the currently open output file
1487when doing edit-in-place processing with B<-i>. Useful when you have
1488to do a lot of inserting and don't want to keep modifying C<$_>. See
1489L<perlrun> for the B<-i> switch.
5b442a2a 1491=item Handle->output_field_separator( EXPR )
84dabc03 1492
1495=item $OFS
1497=item $,
1500The output field separator for the print operator. If defined, this
1501value is printed between each of print's arguments. Default is C<undef>.
1503Mnemonic: what is printed when there is a "," in your print statement.
5b442a2a 1505=item HANDLE->input_line_number( EXPR )
b0c22438 1506
1509=item $NR
1511=item $.
1512X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1514Current line number for the last filehandle accessed.
1516Each filehandle in Perl counts the number of lines that have been read
7333b1c4 1517from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1518constitutes a line may not match yours.) When a line is read from a
1519filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1520C<seek()> is called on it, C<$.> becomes an alias to the line counter
1521for that filehandle.
1523You can adjust the counter by assigning to C<$.>, but this will not
1524actually move the seek pointer. I<Localizing C<$.> will not localize
1525the filehandle's line count>. Instead, it will localize perl's notion
1526of which filehandle C<$.> is currently aliased to.
1528C<$.> is reset when the filehandle is closed, but B<not> when an open
1529filehandle is reopened without an intervening C<close()>. For more
1530details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1531an explicit close, line numbers increase across C<ARGV> files (but see
1532examples in L<perlfunc/eof>).
1534You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1535line counter for a given filehandle without having to worry about
1536which handle you last accessed.
1538Mnemonic: many programs use "." to mean the current line number.
5b442a2a 1540=item HANDLE->input_record_separator( EXPR )
b0c22438 1541
1544=item $RS
1546=item $/
84dabc03 1549The input record separator, newline by default. This influences Perl's
7333b1c4 1550idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1551treating empty lines as a terminator if set to the null string (an
1552empty line cannot contain any spaces or tabs). You may set it to a
1553multi-character string to match a multi-character terminator, or to
1554C<undef> to read through the end of file. Setting it to C<"\n\n">
1555means something slightly different than setting to C<"">, if the file
1556contains consecutive empty lines. Setting to C<""> will treat two or
1557more consecutive empty lines as a single empty line. Setting to
1558C<"\n\n"> will blindly assume that the next input character belongs to
1559the next paragraph, even if it's a newline.
b0c22438 1560
1561 local $/; # enable "slurp" mode
1562 local $_ = <FH>; # whole file now here
1563 s/\n[ \t]+/ /g;
7333b1c4 1565Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1566be better for something. :-)
1568Setting C<$/> to a reference to an integer, scalar containing an
1569integer, or scalar that's convertible to an integer will attempt to
1570read records instead of lines, with the maximum record size being the
1571referenced integer. So this:
1573 local $/ = \32768; # or \"32768", or \$var_containing_32768
1574 open my $fh, "<", $myfile or die $!;
1575 local $_ = <$fh>;
fb73857a 1576
7333b1c4 1577will read a record of no more than 32768 bytes from FILE. If you're
b0c22438 1578not reading from a record-oriented file (or your OS doesn't have
1579record-oriented files), then you'll likely get a full chunk of data
7333b1c4 1580with every read. If a record is larger than the record size you've
1581set, you'll get the record back in pieces. Trying to set the record
b0c22438 1582size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1583
b0c22438 1584On VMS, record reads are done with the equivalent of C<sysread>,
1585so it's best not to mix record and non-record reads on the same
5b442a2a 1586file. (This is unlikely to be a problem, because any file you'd
b0c22438 1587want to read in record mode is probably unusable in line mode.)
1588Non-VMS systems do normal I/O, so it's safe to mix record and
1589non-record reads of a file.
5c055ba3 1590
7333b1c4 1591See also L<perlport/"Newlines">. Also see C<$.>.
9bf22702 1592
b0c22438 1593Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1594
5b442a2a 1595=item Handle->output_record_separator( EXPR )
84902520 1596
b0c22438 1597=item $OUTPUT_RECORD_SEPARATOR
84902520 1598
b0c22438 1599=item $ORS
84902520 1600
b0c22438 1601=item $\
84902520 1603
b0c22438 1604The output record separator for the print operator. If defined, this
1605value is printed after the last of print's arguments. Default is C<undef>.
84902520 1606
b0c22438 1607Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1608Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1609
5b442a2a 1610=item HANDLE->autoflush( EXPR )
84dabc03 1614=item $|
1615X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1616
84dabc03 1617If set to nonzero, forces a flush right away and after every write or
7333b1c4 1618print on the currently selected output channel. Default is 0
84dabc03 1619(regardless of whether the channel is really buffered by the system or
1620not; C<$|> tells you only whether you've asked Perl explicitly to
1621flush after each write). STDOUT will typically be line buffered if
5b442a2a 1622output is to the terminal and block buffered otherwise. Setting this
84dabc03 1623variable is useful primarily when you are outputting to a pipe or
1624socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1625want to see the output as it's happening. This has no effect on input
7333b1c4 1626buffering. See L<perlfunc/getc> for that. See L<perldoc/select> on
84dabc03 1627how to select the output channel. See also L<IO::Handle>.
1629Mnemonic: when you want your pipes to be piping hot.
84902520 1632
b0c22438 1633=head3 Variables related to formats
83ee9e09 1634
b0c22438 1635The special variables for formats are a subset of those for
69b55ccc 1636filehandles. See L<perlform> for more information about Perl's
83ee9e09 1638
b0c22438 1639=over 8
83ee9e09 1640
84dabc03 1641=item $ACCUMULATOR
1643=item $^A
1646The current value of the C<write()> accumulator for C<format()> lines.
1647A format contains C<formline()> calls that put their result into
7333b1c4 1648C<$^A>. After calling its format, C<write()> prints out the contents
84dabc03 1649of C<$^A> and empties. So you never really see the contents of C<$^A>
1650unless you call C<formline()> yourself and then look at it. See
1651L<perlform> and L<perlfunc/formline()>.
5b442a2a 1653=item HANDLE->format_formfeed(EXPR)
84dabc03 1657=item $^L
1660What formats output as a form feed. The default is C<\f>.
b0c22438 1662=item HANDLE->format_page_number(EXPR)
83ee9e09 1663
b0c22438 1664=item $FORMAT_PAGE_NUMBER
83ee9e09 1665
b0c22438 1666=item $%
83ee9e09 1668
b0c22438 1669The current page number of the currently selected output channel.
83ee9e09 1670
b0c22438 1671Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1672
b0c22438 1673=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1674
b0c22438 1675=item $FORMAT_LINES_LEFT
66558a10 1676
b0c22438 1677=item $-
fb73857a 1679
b0c22438 1680The number of lines left on the page of the currently selected output
fa05a9fd 1682
b0c22438 1683Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1684
84dabc03 1685=item Handle->format_line_break_characters EXPR
fb73857a 1686
a0d0e21e 1688
84dabc03 1689=item $:
a0d0e21e 1691
84dabc03 1692The current set of characters after which a string may be broken to
1693fill continuation fields (starting with C<^>) in a format. The default is
1694S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1695
84dabc03 1696Mnemonic: a "colon" in poetry is a part of a line.
1698=item HANDLE->format_lines_per_page(EXPR)
1702=item $=
1705The current page length (printable lines) of the currently selected
1706output channel. The default is 60.
1708Mnemonic: = has horizontal lines.
7c36658b 1709
b0c22438 1710=item HANDLE->format_top_name(EXPR)
7c36658b 1711
b0c22438 1712=item $FORMAT_TOP_NAME
a05d7ebb 1713
b0c22438 1714=item $^
fde18df1 1716
b0c22438 1717The name of the current top-of-page format for the currently selected
1718output channel. The default is the name of the filehandle with C<_TOP>
1719appended. For example, the default format top name for the C<STDOUT>
1720filehanlde is C<STDOUT_TOP>.
e07ea26a 1721
b0c22438 1722Mnemonic: points to top of page.
e07ea26a 1723
84dabc03 1724=item HANDLE->format_name(EXPR)
16070b82 1725
84dabc03 1726=item $FORMAT_NAME
aa2f2a36 1727
84dabc03 1728=item $~
1729X<$~> X<$FORMAT_NAME>
aa2f2a36 1730
84dabc03 1731The name of the current report format for the currently selected
1732output channel. The default format name is the same as the filehandle
1733name. For example, the default format name for the C<STDOUT>
1734filehandle is just C<STDOUT>.
16070b82 1735
84dabc03 1736Mnemonic: brother to C<$^>.
16070b82 1737
b0c22438 1738=back
a0d0e21e 1739
84dabc03 1740=head2 Error Variables
b0c22438 1741X<error> X<exception>
a0d0e21e 1742
b0c22438 1743The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1744about different types of error conditions that may appear during
1745execution of a Perl program. The variables are shown ordered by
1746the "distance" between the subsystem which reported the error and
1747the Perl process. They correspond to errors detected by the Perl
1748interpreter, C library, operating system, or an external program,
4438c4b7 1750
b0c22438 1751To illustrate the differences between these variables, consider the
7fd683ff 1752following Perl expression, which uses a single-quoted string. After
1753execution of this statement, perl may have set all four special error
7333b1c4 1754variables:
4438c4b7 1755
ca1b95ae 1756 eval q{
7333b1c4 1757 open my $pipe, "/cdrom/install |" or die $!;
1758 my @res = <$pipe>;
1759 close $pipe or die "bad pipe: $?, $!";
1760 };
a0d0e21e 1761
7333b1c4 1762When perl executes the C<eval()> expression, it translates the
1763C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
69b55ccc 1764and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1765the C library's C<errno> if one of these calls fails.
2a8c8378 1766
84dabc03 1767C<$@> is set if the string to be C<eval>-ed did not compile (this may
1768happen if C<open> or C<close> were imported with bad prototypes), or
7333b1c4 1769if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1770value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1771will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1772
84dabc03 1773Under a few operating systems, C<$^E> may contain a more verbose error
1774indicator, such as in this case, "CDROM tray not closed." Systems that
1775do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1776
b0c22438 1777Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1778F</cdrom/install> fails. The upper eight bits reflect specific error
1779conditions encountered by the program (the program's C<exit()> value).
1780The lower eight bits reflect mode of failure, like signal death and
1781core dump information. See C<wait(2)> for details. In contrast to
1782C<$!> and C<$^E>, which are set only if error condition is detected,
1783the variable C<$?> is set on each C<wait> or pipe C<close>,
1784overwriting the old value. This is more like C<$@>, which on every
1785C<eval()> is always set on failure and cleared on success.
a0d0e21e 1786
b0c22438 1787For more details, see the individual descriptions at C<$@>, C<$!>,
1788C<$^E>, and C<$?>.
38e4f4ae 1789
0b9346e6 1790=over 8
b0c22438 1792=item ${^CHILD_ERROR_NATIVE}
a0d0e21e 1794
b0c22438 1795The native status returned by the last pipe close, backtick (C<``>)
1796command, successful call to C<wait()> or C<waitpid()>, or from the
1797C<system()> operator. On POSIX-like systems this value can be decoded
1799WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1800
b0c22438 1801Under VMS this reflects the actual VMS exit status; i.e. it is the
1802same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1803
b0c22438 1804This variable was added in Perl 5.8.9.
a0d0e21e 1805
5b442a2a 1806=item $EXTENDED_OS_ERROR
84dabc03 1808=item $^E
1811Error information specific to the current operating system. At the
1812moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1813for MacPerl). On all other platforms, C<$^E> is always just the same
1814as C<$!>.
1816Under VMS, C<$^E> provides the VMS status value from the last system
1817error. This is more specific information about the last system error
1818than that provided by C<$!>. This is particularly important when C<$!>
1819is set to B<EVMSERR>.
1821Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1822API either via CRT, or directly from perl.
1824Under Win32, C<$^E> always returns the last error information reported
1825by the Win32 call C<GetLastError()> which describes the last error
1826from within the Win32 API. Most Win32-specific code will report errors
1827via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1828portable Perl code will report errors via C<$!>.
1830Caveats mentioned in the description of C<$!> generally apply to
1831C<$^E>, also.
1833This variable was added in Perl 5.003.
1835Mnemonic: Extra error explanation.
0b9346e6 1836
84dabc03 1837=item $EXCEPTIONS_BEING_CAUGHT
1839=item $^S
1842Current state of the interpreter.
ca1b95ae 1844 $^S State
1845 --------- -------------------
1846 undef Parsing module/eval
1847 true (1) Executing an eval
1848 false (0) Otherwise
84dabc03 1849
1850The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1853This variable was added in Perl 5.004.
1855=item $WARNING
1857=item $^W
1858X<$^W> X<$WARNING>
1860The current value of the warning switch, initially true if B<-w> was
1861used, false otherwise, but directly modifiable.
1863See also L<warnings>.
0b9346e6 1865Mnemonic: related to the B<-w> switch.
84dabc03 1866
1867=item ${^WARNING_BITS}
ca1b95ae 1868X<${^WARNING_BITS}>
84dabc03 1869
1870The current set of warning checks enabled by the C<use warnings> pragma.
1871See the documentation of C<warnings> for more details.
1873This variable was added in Perl 5.10.
b0c22438 1875=item $OS_ERROR
5ccee41e 1876
b0c22438 1877=item $ERRNO
5ccee41e 1878
b0c22438 1879=item $!
1880X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1881
b0c22438 1882If used numerically, yields the current value of the C C<errno>
1883variable, or in other words, if a system or library call fails, it
1884sets this variable. This means that the value of C<$!> is meaningful
1885only I<immediately> after a B<failure>:
9b0e6e7a 1886
ca1b95ae 1887 if (open my $fh, "<", $filename) {
1888 # Here $! is meaningless.
1889 ...
7fd683ff 1890 }
ca1b95ae 1891 else {
1892 # ONLY here is $! meaningful.
1893 ...
1894 # Already here $! might be meaningless.
b0c22438 1895 }
1896 # Since here we might have either success or failure,
1897 # here $! is meaningless.
a0d0e21e 1898
7333b1c4 1899The I<meaningless> stands for anything: zero, non-zero,
84dabc03 1900C<undef>. A successful system or library call does B<not> set the
1901variable to zero.
a0d0e21e 1902
84dabc03 1903If used as a string, yields the corresponding system error string. You
1904can assign a number to C<$!> to set I<errno> if, for instance, you
1905want C<"$!"> to return the string for error I<n>, or you want to set
1906the exit value for the C<die()> operator.
d54b56d5 1907
b0c22438 1908Mnemonic: What just went bang?
314d39ce 1909
b0c22438 1910=item %OS_ERROR
fb73857a 1911
b0c22438 1912=item %ERRNO
fb73857a 1913
b0c22438 1914=item %!
5b442a2a 1915X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1916
b0c22438 1917Each element of C<%!> has a true value only if C<$!> is set to that
1918value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1919value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1920such file or directory" (or its moral equivalent: not all operating
1921systems give that exact error, and certainly not all languages). To
1922check if a particular key is meaningful on your system, use C<exists
1923$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1924for more information, and also see L</$!>.
a0d0e21e 1925
b0c22438 1926This variable was added in Perl 5.005.
44f0be63 1927
84dabc03 1928=item $CHILD_ERROR
b687b08b 1929
84dabc03 1930=item $?
1931X<$?> X<$CHILD_ERROR>
a0d0e21e 1932
84dabc03 1933The status returned by the last pipe close, backtick (C<``>) command,
1934successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1935operator. This is just the 16-bit status word returned by the
1936traditional Unix C<wait()> system call (or else is made up to look
1937like it). Thus, the exit value of the subprocess is really (C<<< $? >>
19388 >>>), and C<$? & 127> gives which signal, if any, the process died
1939from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1940
84dabc03 1941Additionally, if the C<h_errno> variable is supported in C, its value
1942is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1943
84dabc03 1944If you have installed a signal handler for C<SIGCHLD>, the
1945value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1946
84dabc03 1947Inside an C<END> subroutine C<$?> contains the value that is going to be
1948given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1949change the exit status of your program. For example:
a0d0e21e 1950
84dabc03 1951 END {
1952 $? = 1 if $? == 255; # die would make it 255
1953 }
a0d0e21e 1954
84dabc03 1955Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1956actual VMS exit status, instead of the default emulation of POSIX
1957status; see L<perlvms/$?> for details.
1959Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1960
b0c22438 1961=item $EVAL_ERROR
f648820c 1962
b0c22438 1963=item $@
1964X<$@> X<$EVAL_ERROR>
a0d0e21e 1965
0b9346e6 1966The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1967the null string, the last C<eval()> parsed and executed correctly
b0c22438 1968(although the operations you invoked may have failed in the normal
a0d0e21e 1970
b0c22438 1971Warning messages are not collected in this variable. You can, however,
1972set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1973described in L</%SIG>.
748a9306 1974
b0c22438 1975Mnemonic: Where was the syntax error "at"?
7f315d2e 1976
b0c22438 1977=back
7f315d2e 1978
b0c22438 1979=head2 Deprecated and removed variables
7f315d2e 1980
0b9346e6 1981Deprecating a variable announces the intent of the perl maintainers to
84dabc03 1982eventually remove the variable from the langauge. It may still be
b0c22438 1983available despite its status. Using a deprecated variable triggers
1984a warning.
7f315d2e 1985
84dabc03 1986Once a variable is removed, its use triggers an error telling you
b0c22438 1987the variable is unsupported.
7f315d2e 1988
84dabc03 1989See L<perldiag> for details about error messages.
7f315d2e 1990
b0c22438 1991=over 8
7f315d2e 1992
5b442a2a 1993=item $OFMT
84dabc03 1995=item $#
5b442a2a 1996X<$#> X<$OFMT>
84dabc03 1997
1998C<$#> was a variable that you could be use to format printed numbers.
1999After a deprecation cycle, its magic was removed in Perl 5.10 and
2000using it now triggers a warning: C<$# is no longer supported>.
2002This is not the sigil you use in front of an array name to get the
2003last index, like C<$#array>. That's still how you get the last index
2004of an array in Perl. The two have nothing to do with each other.
2006Deprecated in Perl 5.
2008Removed in Perl 5.10.
2010=item $*
84dabc03 2013C<$*> was a variable that you could use to enable multiline matching.
2014After a deprecation cycle, its magic was removed in Perl 5.10.
2015Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2016You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2017
b0c22438 2018Deprecated in Perl 5.
7f315d2e 2019
b0c22438 2020Removed in Perl 5.10.
7f315d2e 2021
5b442a2a 2022=item $ARRAY_BASE
84dabc03 2024=item $[
5b442a2a 2025X<$[> X<$ARRAY_BASE>
84dabc03 2026
2027This variable stores the index of the first element in an array, and
2028of the first character in a substring. You use to be able to assign to
2029this variable, but you can't do that anymore. It's now always 0, like
0b9346e6 2030it should be.
84dabc03 2031
2032Mnemonic: [ begins subscripts.
2034This variable is read-only.
0b9346e6 2036Deprecated in Perl 5.12.
84dabc03 2037
5b442a2a 2038=item $OLD_PERL_VERSION
b0c22438 2040=item $]
5b442a2a 2041X<$]> X<$OLD_PERL_VERSION>
55602bd2 2042
d4ba9bf2 2043See C<$^V> for a more modern representation of the Perl version that allows
2044accurate string comparisons.
b0c22438 2046The version + patchlevel / 1000 of the Perl interpreter. This variable
2047can be used to determine whether the Perl interpreter executing a
2048script is in the right range of versions:
55602bd2 2049
b0c22438 2050 warn "No checksumming!\n" if $] < 3.019;
55602bd2 2051
d4ba9bf2 2052The floating point representation can sometimes lead to inaccurate
2053numeric comparisons.
b0c22438 2055See also the documentation of C<use VERSION> and C<require VERSION>
2056for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 2057
b0c22438 2058Mnemonic: Is this version of perl in the right bracket?
19799a22 2059
b0c22438 2060Deprecated in Perl 5.6.
19799a22 2061
b0c22438 2062=back
2b92dfce 2063
0b9346e6 2064=cut