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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'>.
b0c22438 447=item $^H
5b442a2a 448X<$^H>
883faa13 449
b0c22438 450WARNING: This variable is strictly for internal use only. Its availability,
451behavior, and contents are subject to change without notice.
a0d0e21e 452
b0c22438 453This variable contains compile-time hints for the Perl interpreter. At the
454end of compilation of a BLOCK the value of this variable is restored to the
455value when the interpreter started to compile the BLOCK.
a0d0e21e 456
b0c22438 457When perl begins to parse any block construct that provides a lexical scope
458(e.g., eval body, required file, subroutine body, loop body, or conditional
459block), the existing value of C<$^H> is saved, but its value is left unchanged.
460When the compilation of the block is completed, it regains the saved value.
461Between the points where its value is saved and restored, code that
462executes within BEGIN blocks is free to change the value of C<$^H>.
a0d0e21e 463
b0c22438 464This behavior provides the semantic of lexical scoping, and is used in,
465for instance, the C<use strict> pragma.
a0d0e21e 466
b0c22438 467The contents should be an integer; different bits of it are used for
468different pragmatic flags. Here's an example:
a0d0e21e 469
0b9346e6 470 sub add_100 { $^H |= 0x100 }
a0d0e21e 471
0b9346e6 472 sub foo {
473 BEGIN { add_100() }
474 bar->baz($boon);
475 }
a0d0e21e 476
b0c22438 477Consider what happens during execution of the BEGIN block. At this point
478the BEGIN block has already been compiled, but the body of C<foo()> is still
479being compiled. The new value of C<$^H> will therefore be visible only while
480the body of C<foo()> is being compiled.
a0d0e21e 481
7333b1c4 482Substitution of C<BEGIN { add_100() }> block with:
a0d0e21e 483
0b9346e6 484 BEGIN { require strict; strict->import('vars') }
a0d0e21e 485
7333b1c4 486demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
b0c22438 487version of the same lexical pragma:
a0d0e21e 488
0b9346e6 489 BEGIN { require strict; strict->import('vars') if $condition }
a0d0e21e 490
b0c22438 491This variable was added in Perl 5.003.
a0d0e21e 492
b0c22438 493=item %^H
5b442a2a 494X<%^H>
a0d0e21e 495
b0c22438 496The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
497useful for implementation of lexically scoped pragmas. See L<perlpragma>.
a0d0e21e 498
b0c22438 499This variable was added in Perl 5.6.
a0d0e21e 500
0b9346e6 501=item @INC
504The array C<@INC> contains the list of places that the C<do EXPR>,
7333b1c4 505C<require>, or C<use> constructs look for their library files. It
0b9346e6 506initially consists of the arguments to any B<-I> command-line
507switches, followed by the default Perl library, probably
508F</usr/local/lib/perl>, followed by ".", to represent the current
7333b1c4 509directory. ("." will not be appended if taint checks are enabled,
0b9346e6 510either by C<-T> or by C<-t>.) If you need to modify this at runtime,
511you should use the C<use lib> pragma to get the machine-dependent
512library properly loaded also:
514 use lib '/mypath/libdir/';
515 use SomeMod;
517You can also insert hooks into the file inclusion system by putting Perl
518code directly into C<@INC>. Those hooks may be subroutine references, array
519references or blessed objects. See L<perlfunc/require> for details.
521=item %INC
524The hash C<%INC> contains entries for each filename included via the
525C<do>, C<require>, or C<use> operators. The key is the filename
526you specified (with module names converted to pathnames), and the
527value is the location of the file found. The C<require>
528operator uses this hash to determine whether a particular file has
529already been included.
531If the file was loaded via a hook (e.g. a subroutine reference, see
532L<perlfunc/require> for a description of these hooks), this hook is
533by default inserted into C<%INC> in place of a filename. Note, however,
534that the hook may have set the C<%INC> entry by itself to provide some more
535specific info.
b0c22438 537=item $INPLACE_EDIT
a0d0e21e 538
b0c22438 539=item $^I
a0d0e21e 541
b0c22438 542The current value of the inplace-edit extension. Use C<undef> to disable
543inplace editing.
a0d0e21e 544
b0c22438 545Mnemonic: value of B<-i> switch.
a0d0e21e 546
b0c22438 547=item $^M
a0d0e21e 549
b0c22438 550By default, running out of memory is an untrappable, fatal error.
551However, if suitably built, Perl can use the contents of C<$^M>
552as an emergency memory pool after C<die()>ing. Suppose that your Perl
553were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
a0d0e21e 555
0b9346e6 556 $^M = 'a' x (1 << 16);
a0d0e21e 557
b0c22438 558would allocate a 64K buffer for use in an emergency. See the
559F<INSTALL> file in the Perl distribution for information on how to
560add custom C compilation flags when compiling perl. To discourage casual
561use of this advanced feature, there is no L<English|English> long name for
562this variable.
a0d0e21e 563
b0c22438 564This variable was added in Perl 5.004.
a0d0e21e 565
b0c22438 566=item $OSNAME
a0d0e21e 567
b0c22438 568=item $^O
569X<$^O> X<$OSNAME>
a0d0e21e 570
b0c22438 571The name of the operating system under which this copy of Perl was
572built, as determined during the configuration process. For examples
573see L<perlport/PLATFORMS>.
a0d0e21e 574
b0c22438 575The value is identical to C<$Config{'osname'}>. See also L<Config>
576and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 577
b0c22438 578In Windows platforms, C<$^O> is not very helpful: since it is always
579C<MSWin32>, it doesn't tell the difference between
58095/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
581Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
582between the variants.
a0d0e21e 583
b0c22438 584This variable was added in Perl 5.003.
a0d0e21e 585
b0c22438 586=item ${^OPEN}
5b442a2a 587X<${^OPEN}>
a0d0e21e 588
b0c22438 589An internal variable used by PerlIO. A string in two parts, separated
590by a C<\0> byte, the first part describes the input layers, the second
591part describes the output layers.
a0d0e21e 592
b0c22438 593This variable was added in Perl 5.8.2.
a0d0e21e 594
b0c22438 595=item $PERLDB
a0d0e21e 596
b0c22438 597=item $^P
598X<$^P> X<$PERLDB>
a0d0e21e 599
b0c22438 600The internal variable for debugging support. The meanings of the
601various bits are subject to change, but currently indicate:
a0d0e21e 602
b0c22438 603=over 6
a0d0e21e 604
b0c22438 605=item 0x01
a0d0e21e 606
b0c22438 607Debug subroutine enter/exit.
a0d0e21e 608
b0c22438 609=item 0x02
a0d0e21e 610
b0c22438 611Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
612statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 613
b0c22438 614=item 0x04
fe307981 615
b0c22438 616Switch off optimizations.
6cef1e77 617
b0c22438 618=item 0x08
6cef1e77 619
b0c22438 620Preserve more data for future interactive inspections.
6cef1e77 621
b0c22438 622=item 0x10
4ba05bdc 623
b0c22438 624Keep info about source lines on which a subroutine is defined.
4ba05bdc 625
b0c22438 626=item 0x20
4ba05bdc 627
b0c22438 628Start with single-step on.
4ba05bdc 629
b0c22438 630=item 0x40
4ba05bdc 631
b0c22438 632Use subroutine address instead of name when reporting.
4ba05bdc 633
b0c22438 634=item 0x80
4ba05bdc 635
b0c22438 636Report C<goto &subroutine> as well.
4ba05bdc 637
b0c22438 638=item 0x100
4ba05bdc 639
b0c22438 640Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 641
b0c22438 642=item 0x200
44a2ac75 643
b0c22438 644Provide informative names to anonymous subroutines based on the place they
645were compiled.
44a2ac75 646
b0c22438 647=item 0x400
44a2ac75 648
b0c22438 649Save source code lines into C<@{"_<$filename"}>.
44a2ac75 650
b0c22438 651=back
44a2ac75 652
b0c22438 653Some bits may be relevant at compile-time only, some at
7333b1c4 654run-time only. This is a new mechanism and the details may change.
b0c22438 655See also L<perldebguts>.
3195cf34 656
b0c22438 657=item %SIG
b0c22438 658X<%SIG>
a0d0e21e 659
b0c22438 660The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 661
0b9346e6 662 sub handler { # 1st argument is signal name
663 my($sig) = @_;
664 print "Caught a SIG$sig--shutting down\n";
665 close(LOG);
666 exit(0);
667 }
a0d0e21e 668
0b9346e6 669 $SIG{'INT'} = \&handler;
670 $SIG{'QUIT'} = \&handler;
671 ...
672 $SIG{'INT'} = 'DEFAULT'; # restore default action
673 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 674
b0c22438 675Using a value of C<'IGNORE'> usually has the effect of ignoring the
676signal, except for the C<CHLD> signal. See L<perlipc> for more about
677this special case.
a0d0e21e 678
b0c22438 679Here are some other examples:
a0d0e21e 680
0b9346e6 681 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
682 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
683 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
684 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 685
b0c22438 686Be sure not to use a bareword as the name of a signal handler,
687lest you inadvertently call it.
a0d0e21e 688
b0c22438 689If your system has the C<sigaction()> function then signal handlers
690are installed using it. This means you get reliable signal handling.
7b8d334a 691
b0c22438 692The default delivery policy of signals changed in Perl 5.8.0 from
693immediate (also known as "unsafe") to deferred, also known as "safe
7333b1c4 694signals". See L<perlipc> for more information.
aa689395 695
b0c22438 696Certain internal hooks can be also set using the C<%SIG> hash. The
697routine indicated by C<$SIG{__WARN__}> is called when a warning
7333b1c4 698message is about to be printed. The warning message is passed as the
699first argument. The presence of a C<__WARN__> hook causes the
b0c22438 700ordinary printing of warnings to C<STDERR> to be suppressed. You can
701use this to save warnings in a variable, or turn warnings into fatal
702errors, like this:
19799a22 703
0b9346e6 704 local $SIG{__WARN__} = sub { die $_[0] };
705 eval $proggie;
a8f8344d 706
b0c22438 707As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
708disable warnings using the empty subroutine:
f86702cc 709
0b9346e6 710 local $SIG{__WARN__} = sub {};
55602bd2 711
b0c22438 712The routine indicated by C<$SIG{__DIE__}> is called when a fatal
713exception is about to be thrown. The error message is passed as the
714first argument. When a C<__DIE__> hook routine returns, the exception
715processing continues as it would have in the absence of the hook,
716unless the hook routine itself exits via a C<goto>, a loop exit, or a
717C<die()>. The C<__DIE__> handler is explicitly disabled during the
718call, so that you can die from a C<__DIE__> handler. Similarly for
e5218da5 720
b0c22438 721Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
722even inside an C<eval()>. Do not use this to rewrite a pending
723exception in C<$@>, or as a bizarre substitute for overriding
724C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
725in a future release so that C<$SIG{__DIE__}> is only called if your
726program is about to exit, as was the original intent. Any other use is
729C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
730may be called to report (probable) errors found by the parser. In such
731a case the parser may be in inconsistent state, so any attempt to
732evaluate Perl code from such a handler will probably result in a
733segfault. This means that warnings or errors that result from parsing
734Perl should be used with extreme caution, like this:
e5218da5 735
0b9346e6 736 require Carp if defined $^S;
737 Carp::confess("Something wrong") if defined &Carp::confess;
738 die "Something wrong, but could not load Carp to give backtrace...
739 To see backtrace try starting Perl with -MCarp switch";
e5218da5 740
b0c22438 741Here the first line will load C<Carp> I<unless> it is the parser who
742called the handler. The second line will print backtrace and die if
743C<Carp> was available. The third line will be executed only if C<Carp> was
744not available.
0a378802 745
0b9346e6 746Having to even think about the C<$^S> variable in your exception
7333b1c4 747handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
0b9346e6 748invites grievous and difficult to track down errors. Avoid it
749and use an C<END{}> or CORE::GLOBAL::die override instead.
b0c22438 751See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
752L<warnings> for additional information.
0a378802 753
b0c22438 754=item $BASETIME
6ab308ee 755
b0c22438 756=item $^T
757X<$^T> X<$BASETIME>
6ab308ee 758
b0c22438 759The time at which the program began running, in seconds since the
760epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
761and B<-C> filetests are based on this value.
a0d0e21e 762
b0c22438 763=item ${^TAINT}
5b442a2a 764X<${^TAINT}>
55602bd2 765
b0c22438 766Reflects if taint mode is on or off. 1 for on (the program was run with
767B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 768B<-t> or B<-TU>).
daaddde1 769
b0c22438 770This variable is read-only.
daaddde1 771
b0c22438 772This variable was added in Perl 5.8.
4c5cef9b 773
b0c22438 774=item ${^UNICODE}
5b442a2a 775X<${^UNICODE}>
4c5cef9b 776
7333b1c4 777Reflects certain Unicode settings of Perl. See L<perlrun>
b0c22438 778documentation for the C<-C> switch for more information about
0b9346e6 779the possible values.
5c055ba3 780
b0c22438 781This variable is set during Perl startup and is thereafter read-only.
5c055ba3 782
b0c22438 783This variable was added in Perl 5.8.2.
22fae026 784
b0c22438 785=item ${^UTF8CACHE}
5b442a2a 786X<${^UTF8CACHE}>
22fae026 787
b0c22438 788This variable controls the state of the internal UTF-8 offset caching code.
7891 for on (the default), 0 for off, -1 to debug the caching code by checking
790all its results against linear scans, and panicking on any discrepancy.
22fae026 791
b0c22438 792This variable was added in Perl 5.8.9.
22fae026 793
b0c22438 794=item ${^UTF8LOCALE}
5b442a2a 795X<${^UTF8LOCALE}>
5c055ba3 796
b0c22438 797This variable indicates whether a UTF-8 locale was detected by perl at
798startup. This information is used by perl when it's in
799adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
800switch); see L<perlrun> for more info on this.
55602bd2 801
b0c22438 802This variable was added in Perl 5.8.8.
a0d0e21e 803
b0c22438 804=item $PERL_VERSION
a0d0e21e 805
b0c22438 806=item $^V
a0d0e21e 808
b0c22438 809The revision, version, and subversion of the Perl interpreter,
810represented as a C<version> object.
748a9306 811
b0c22438 812This variable first appeared in perl 5.6.0; earlier versions of perl
813will see an undefined value. Before perl 5.10.0 C<$^V> was represented
814as a v-string.
55602bd2 815
b0c22438 816C<$^V> can be used to determine whether the Perl interpreter executing
817a script is in the right range of versions. For example:
a0d0e21e 818
0b9346e6 819 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 820
b0c22438 821To convert C<$^V> into its string representation use C<sprintf()>'s
822C<"%vd"> conversion:
a0d0e21e 823
0b9346e6 824 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 825
b0c22438 826See the documentation of C<use VERSION> and C<require VERSION>
827for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 828
b0c22438 829See also C<$]> for an older representation of the Perl version.
a0d0e21e 830
b0c22438 831This variable was added in Perl 5.6.
a0d0e21e 832
b0c22438 833Mnemonic: use ^V for Version Control.
a0d0e21e 834
b0c22438 835=item ${^WIN32_SLOPPY_STAT}
5b442a2a 836X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<>
a0d0e21e 837
b0c22438 838If this variable is set to a true value, then C<stat()> on Windows will
839not try to open the file. This means that the link count cannot be
840determined and file attributes may be out of date if additional
841hardlinks to the file exist. On the other hand, not opening the file
842is considerably faster, especially for files on network drives.
a0d0e21e 843
b0c22438 844This variable could be set in the F<> file to
845configure the local Perl installation to use "sloppy" C<stat()> by
846default. See the documentation for B<-f> in
847L<perlrun|perlrun/"Command Switches"> for more information about site
a0d0e21e 849
b0c22438 850This variable was added in Perl 5.10.
a0d0e21e 851
b0c22438 852=item $EXECUTABLE_NAME
a0d0e21e 853
b0c22438 854=item $^X
a0d0e21e 856
b0c22438 857The name used to execute the current copy of Perl, from C's
858C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 859
b0c22438 860Depending on the host operating system, the value of C<$^X> may be
861a relative or absolute pathname of the perl program file, or may
862be the string used to invoke perl but not the pathname of the
863perl program file. Also, most operating systems permit invoking
864programs that are not in the PATH environment variable, so there
865is no guarantee that the value of C<$^X> is in PATH. For VMS, the
866value may or may not include a version number.
a0d0e21e 867
b0c22438 868You usually can use the value of C<$^X> to re-invoke an independent
869copy of the same perl that is currently running, e.g.,
a0d0e21e 870
0b9346e6 871 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 872
b0c22438 873But recall that not all operating systems support forking or
874capturing of the output of commands, so this complex statement
875may not be portable.
a0d0e21e 876
b0c22438 877It is not safe to use the value of C<$^X> as a path name of a file,
878as some operating systems that have a mandatory suffix on
879executable files do not require use of the suffix when invoking
880a command. To convert the value of C<$^X> to a path name, use the
881following statements:
8cc95fdb 882
0b9346e6 883 # Build up a set of file names (not command names).
884 use Config;
885 my $this_perl = $^X;
886 if ($^O ne 'VMS') {
887 $this_perl .= $Config{_exe}
888 unless $this_perl =~ m/$Config{_exe}$/i;
889 }
8cc95fdb 890
b0c22438 891Because many operating systems permit anyone with read access to
892the Perl program file to make a copy of it, patch the copy, and
893then execute the copy, the security-conscious Perl programmer
894should take care to invoke the installed copy of perl, not the
895copy referenced by C<$^X>. The following statements accomplish
896this goal, and produce a pathname that can be invoked as a
897command or referenced as a file.
a043a685 898
0b9346e6 899 use Config;
900 my $secure_perl_path = $Config{perlpath};
901 if ($^O ne 'VMS') {
902 $secure_perl_path .= $Config{_exe}
903 unless $secure_perl_path =~ m/$Config{_exe}$/i;
904 }
a0d0e21e 905
b0c22438 906=back
a0d0e21e 907
b0c22438 908=head2 Variables related to regular expressions
910Most of the special variables related to regular expressions are side
911effects. Perl sets these variables when it has a successful match, so
912you should check the match result before using them. For instance:
914 if( /P(A)TT(ER)N/ ) {
915 print "I found $1 and $2\n";
916 }
0b9346e6 918These variables are read-only and dynamically-scoped, unless we note
b0c22438 919otherwise.
0b9346e6 921The dynamic nature of the regular expression variables means that
922their value is limited to the block that they are in, as demonstrated
923by this bit of code:
b0c22438 924
925 my $outer = 'Wallace and Grommit';
926 my $inner = 'Mutt and Jeff';
0b9346e6 927
b0c22438 928 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 929
b0c22438 930 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 931
b0c22438 932 {
933 OUTER:
934 show_n() if $outer =~ m/$pattern/;
0b9346e6 935
b0c22438 936 INNER: {
937 show_n() if $inner =~ m/$pattern/;
938 }
0b9346e6 939
b0c22438 940 show_n();
941 }
0b9346e6 943The output shows that while in the C<OUTER> block, the values of C<$1>
944and C<$2> are from the match against C<$outer>. Inside the C<INNER>
945block, the values of C<$1> and C<$2> are from the match against
946C<$inner>, but only until the end of the block (i.e. the dynamic
947scope). After the C<INNER> block completes, the values of C<$1> and
948C<$2> return to the values for the match against C<$outer> even though
b0c22438 949we have not made another match:
951 $1 is Wallace; $2 is Grommit
952 $1 is Mutt; $2 is Jeff
953 $1 is Wallace; $2 is Grommit
a0d0e21e 954
0b9346e6 955Due to an unfortunate accident of Perl's implementation, C<use
956English> imposes a considerable performance penalty on all regular
957expression matches in a program because it uses the C<$`>, C<$&>, and
958C<$'>, regardless of whether they occur in the scope of C<use
959English>. For that reason, saying C<use English> in libraries is
960strongly discouraged unless you import it without the match variables:
962 use English '-no_match_vars'
964The C<Devel::NYTProf> module can help you find uses of these
965problematic match variables in your code.
967Since Perl 5.10, you can use the C</p> match operator flag and the
968C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
969so you only suffer the performance penalties.
b0c22438 971=over 8
a0d0e21e 972
b0c22438 973=item $<I<digits>> ($1, $2, ...)
974X<$1> X<$2> X<$3>
8cc95fdb 975
b0c22438 976Contains the subpattern from the corresponding set of capturing
977parentheses from the last successful pattern match, not counting patterns
978matched in nested blocks that have been exited already.
8cc95fdb 979
b0c22438 980These variables are read-only and dynamically-scoped.
a043a685 981
b0c22438 982Mnemonic: like \digits.
a0d0e21e 983
b0c22438 984=item $MATCH
a0d0e21e 985
b0c22438 986=item $&
987X<$&> X<$MATCH>
a0d0e21e 988
b0c22438 989The string matched by the last successful pattern match (not counting
990any matches hidden within a BLOCK or C<eval()> enclosed by the current
a0d0e21e 992
b0c22438 993The use of this variable anywhere in a program imposes a considerable
0b9346e6 994performance penalty on all regular expression matches. To avoid this
995penalty, you can extract the same substring by using L</@->. Starting
996with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
997variable to do the same thing for particular match operations.
80bca1b4 998
b0c22438 999This variable is read-only and dynamically-scoped.
f9cbb277 1000
b0c22438 1001Mnemonic: like C<&> in some editors.
0b9346e6 1002
b0c22438 1003=item ${^MATCH}
a0d0e21e 1005
b0c22438 1006This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1007performance penalty associated with that variable, and is only guaranteed
1008to return a defined value when the pattern was compiled or executed with
1009the C</p> modifier.
80bca1b4 1010
b0c22438 1011This variable was added in Perl 5.10.
4bc88a62 1012
b0c22438 1013This variable is read-only and dynamically-scoped.
e2975953 1014
b0c22438 1015=item $PREMATCH
52c447a8 1016
b0c22438 1017=item $`
5b442a2a 1018X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 1019
b0c22438 1020The string preceding whatever was matched by the last successful
1021pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1022enclosed by the current BLOCK.
a0d0e21e 1023
b0c22438 1024The use of this variable anywhere in a program imposes a considerable
0b9346e6 1025performance penalty on all regular expression matches. To avoid this
1026penalty, you can extract the same substring by using L</@->. Starting
1027with Perl 5.10, you can use the </p> match flag and the
1028C<${^PREMATCH}> variable to do the same thing for particular match
a0d0e21e 1030
b0c22438 1031This variable is read-only and dynamically-scoped.
a0d0e21e 1032
b0c22438 1033Mnemonic: C<`> often precedes a quoted string.
f83ed198 1034
b0c22438 1035=item ${^PREMATCH}
5b442a2a 1036X<$`> X<${^PREMATCH}>
a0d0e21e 1037
b0c22438 1038This is similar to C<$`> ($PREMATCH) except that it does not incur the
1039performance penalty associated with that variable, and is only guaranteed
1040to return a defined value when the pattern was compiled or executed with
1041the C</p> modifier.
a0d0e21e 1042
b0c22438 1043This variable was added in Perl 5.10
a0d0e21e 1044
b0c22438 1045This variable is read-only and dynamically-scoped.
a0d0e21e 1046
b0c22438 1047=item $POSTMATCH
16070b82 1048
b0c22438 1049=item $'
5b442a2a 1050X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1051
b0c22438 1052The string following whatever was matched by the last successful
1053pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1054enclosed by the current BLOCK). Example:
305aace0 1055
0b9346e6 1056 local $_ = 'abcdefghi';
1057 /def/;
1058 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1059
b0c22438 1060The use of this variable anywhere in a program imposes a considerable
0b9346e6 1061performance penalty on all regular expression matches.
1062To avoid this penalty, you can extract the same substring by
b0c22438 1063using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1064and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1065match operations.
a0d0e21e 1066
b0c22438 1067This variable is read-only and dynamically-scoped.
1069Mnemonic: C<'> often follows a quoted string.
1071=item ${^POSTMATCH}
5b442a2a 1072X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1073
1074This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1075performance penalty associated with that variable, and is only guaranteed
1076to return a defined value when the pattern was compiled or executed with
1077the C</p> modifier.
1079This variable was added in Perl 5.10.
1081This variable is read-only and dynamically-scoped.
1085=item $+
1088The text matched by the last bracket of the last successful search pattern.
1089This is useful if you don't know which one of a set of alternative patterns
1090matched. For example:
0b9346e6 1092 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1093
1094This variable is read-only and dynamically-scoped.
1096Mnemonic: be positive and forward looking.
1100=item $^N
5b442a2a 1101X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1102
1103The text matched by the used group most-recently closed (i.e. the group
1104with the rightmost closing parenthesis) of the last successful search
1107This is primarily used inside C<(?{...})> blocks for examining text
1108recently matched. For example, to effectively capture text to a variable
1109(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
0b9346e6 1111 (?:(...)(?{ $var = $^N }))
b0c22438 1112
1113By setting and then using C<$var> in this way relieves you from having to
1114worry about exactly which numbered set of parentheses they are.
1116This variable was added in Perl 5.8.
1118Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1120=item @LAST_MATCH_END
1122=item @+
1123X<@+> X<@LAST_MATCH_END>
1125This array holds the offsets of the ends of the last successful
1126submatches in the currently active dynamic scope. C<$+[0]> is
1127the offset into the string of the end of the entire match. This
1128is the same value as what the C<pos> function returns when called
1129on the variable that was matched against. The I<n>th element
1130of this array holds the offset of the I<n>th submatch, so
1131C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
7333b1c4 1132past where C<$2> ends, and so on. You can use C<$#+> to determine
b0c22438 1133how many subgroups were in the last successful match. See the
1134examples given for the C<@-> variable.
1136This variable was added in Perl 5.6.
1140=item %+
5b442a2a 1141X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1142
1143Similar to C<@+>, the C<%+> hash allows access to the named capture
1144buffers, should they exist, in the last successful match in the
1145currently active dynamic scope.
1147For example, C<$+{foo}> is equivalent to C<$1> after the following match:
0b9346e6 1149 'foo' =~ /(?<foo>foo)/;
b0c22438 1150
1151The keys of the C<%+> hash list only the names of buffers that have
1152captured (and that are thus associated to defined values).
1154The underlying behaviour of C<%+> is provided by the
1155L<Tie::Hash::NamedCapture> module.
1157B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1158associated with the last successful regular expression. Therefore mixing
1159iterative access to them via C<each> may have unpredictable results.
1160Likewise, if the last successful match changes, then the results may be
1163This variable was added in Perl 5.10.
a0d0e21e 1164
b0c22438 1165This variable is read-only and dynamically-scoped.
1169=item @-
1172C<$-[0]> is the offset of the start of the last successful match.
1173C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1174I<n>-th subpattern, or undef if the subpattern did not match.
1176Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1177$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1178$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1179C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1180matched subgroup in the last successful match. Contrast with
1181C<$#+>, the number of subgroups in the regular expression. Compare
1182with C<@+>.
1184This array holds the offsets of the beginnings of the last
1185successful submatches in the currently active dynamic scope.
1186C<$-[0]> is the offset into the string of the beginning of the
7333b1c4 1187entire match. The I<n>th element of this array holds the offset
b0c22438 1188of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1189begins, C<$-[2]> the offset where C<$2> begins, and so on.
1191After a match against some variable C<$var>:
1193=over 5
1195=item C<$`> is the same as C<substr($var, 0, $-[0])>
1197=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1199=item C<$'> is the same as C<substr($var, $+[0])>
1201=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1203=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1205=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1209This variable was added in Perl 5.6.
5b442a2a 1211=item %LAST_MATCH_START
b0c22438 1213=item %-
5b442a2a 1214X<%-> X<%LAST_MATCH_START>
b0c22438 1215
1216Similar to C<%+>, this variable allows access to the named capture groups
1217in the last successful match in the currently active dynamic scope. To
1218each capture group name found in the regular expression, it associates a
1219reference to an array containing the list of values captured by all
1220buffers with that name (should there be several of them), in the order
1221where they appear.
1223Here's an example:
1225 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1226 foreach my $bufname (sort keys %-) {
1227 my $ary = $-{$bufname};
1228 foreach my $idx (0..$#$ary) {
1229 print "\$-{$bufname}[$idx] : ",
1230 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1231 "\n";
1232 }
1233 }
1234 }
1236would print out:
0b9346e6 1238 $-{A}[0] : '1'
1239 $-{A}[1] : '3'
1240 $-{B}[0] : '2'
1241 $-{B}[1] : '4'
b0c22438 1242
1243The keys of the C<%-> hash correspond to all buffer names found in
1244the regular expression.
1246The behaviour of C<%-> is implemented via the
1247L<Tie::Hash::NamedCapture> module.
1249B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1250associated with the last successful regular expression. Therefore mixing
1251iterative access to them via C<each> may have unpredictable results.
1252Likewise, if the last successful match changes, then the results may be
1255This variable was added in Perl 5.10
1257This variable is read-only and dynamically-scoped.
1261=item $^R
1264The result of evaluation of the last successful C<(?{ code })>
1265regular expression assertion (see L<perlre>). May be written to.
1267This variable was added in Perl 5.005.
a0d0e21e 1268
a3621e74 1269=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1270X<${^RE_DEBUG_FLAGS}>
1272The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1273even when the C<re 'debug'> module is loaded. See L<re> for details.
1275This variable was added in Perl 5.10.
a3621e74 1276
0111c4fd 1277=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1278X<${^RE_TRIE_MAXBUF}>
1280Controls how certain regex optimisations are applied and how much memory they
1281utilize. This value by default is 65536 which corresponds to a 512kB temporary
1282cache. Set this to a higher value to trade memory for speed when matching
1283large alternations. Set it to a lower value if you want the optimisations to
1284be as conservative of memory as possible but still occur, and set it to a
1285negative value to prevent the optimisation and conserve the most memory.
1286Under normal situations this variable should be of no interest to you.
b0c22438 1288This variable was added in Perl 5.10.
a0d0e21e 1289
b0c22438 1290=back
a0d0e21e 1291
b0c22438 1292=head2 Variables related to filehandles
a0d0e21e 1293
b0c22438 1294Variables that depend on the currently selected filehandle may be set
1295by calling an appropriate object method on the C<IO::Handle> object,
1296although this is less efficient than using the regular built-in
1297variables. (Summary lines below for this contain the word HANDLE.)
1298First you must say
6e2995f4 1299
0b9346e6 1300 use IO::Handle;
0462a1ab 1301
b0c22438 1302after which you may use either
0462a1ab 1303
0b9346e6 1304 method HANDLE EXPR
0462a1ab 1305
b0c22438 1306or more safely,
0462a1ab 1307
0b9346e6 1308 HANDLE->method(EXPR)
0462a1ab 1309
b0c22438 1310Each method returns the old value of the C<IO::Handle> attribute. The
1311methods each take an optional EXPR, which, if supplied, specifies the
1312new value for the C<IO::Handle> attribute in question. If not
1313supplied, most methods do nothing to the current value--except for
1314C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1315
b0c22438 1316Because loading in the C<IO::Handle> class is an expensive operation,
1317you should learn how to use the regular built-in variables.
1319A few of these variables are considered "read-only". This means that
1320if you try to assign to this variable, either directly or indirectly
1321through a reference, you'll raise a run-time exception.
1323You should be very careful when modifying the default values of most
1324special variables described in this document. In most cases you want
1325to localize these variables before changing them, since if you don't,
1326the change may affect other modules which rely on the default values
1327of the special variables that you have changed. This is one of the
1328correct ways to read the whole file at once:
0b9346e6 1330 open my $fh, "<", "foo" or die $!;
1331 local $/; # enable localized slurp mode
1332 my $content = <$fh>;
1333 close $fh;
b0c22438 1334
1335But the following code is quite bad:
0b9346e6 1337 open my $fh, "<", "foo" or die $!;
1338 undef $/; # enable slurp mode
1339 my $content = <$fh>;
1340 close $fh;
b0c22438 1341
1342since some other module, may want to read data from some file in the
1343default "line mode", so if the code we have just presented has been
1344executed, the global value of C<$/> is now changed for any other code
1345running inside the same Perl interpreter.
1347Usually when a variable is localized you want to make sure that this
1348change affects the shortest scope possible. So unless you are already
1349inside some short C<{}> block, you should create one yourself. For
0b9346e6 1352 my $content = '';
1353 open my $fh, "<", "foo" or die $!;
1354 {
1355 local $/;
1356 $content = <$fh>;
1357 }
1358 close $fh;
0462a1ab 1359
b0c22438 1360Here is an example of how your own code can go broken:
0462a1ab 1361
0b9346e6 1362 for ( 1..3 ){
1363 $\ = "\r\n";
1364 nasty_break();
1365 print "$_";
1366 }
1368 sub nasty_break {
1369 $\ = "\f";
1370 # do something with $_
1371 }
0462a1ab 1372
0b9346e6 1373You probably expect this code to print the equivalent of
0462a1ab 1374
0b9346e6 1375 "1\r\n2\r\n3\r\n"
0462a1ab 1376
b0c22438 1377but instead you get:
0462a1ab 1378
0b9346e6 1379 "1\f2\f3\f"
0462a1ab 1380
0b9346e6 1381Why? Because C<nasty_break()> modifies C<$\> without localizing it
1382first. The value you set in C<nasty_break()> is still there when you
1383return. The fix is to add C<local()> so the value doesn't leak out of
6e2995f4 1385
0b9346e6 1386 local $\ = "\f";
a0d0e21e 1387
b0c22438 1388It's easy to notice the problem in such a short example, but in more
1389complicated code you are looking for trouble if you don't localize
1390changes to the special variables.
a0d0e21e 1391
b0c22438 1392=over 8
a0d0e21e 1393
b0c22438 1394=item $ARGV
fb73857a 1396
ca1b95ae 1397Contains the name of the current file when reading from C<< <> >>.
b0c22438 1398
1399=item @ARGV
ca1b95ae 1402The array C<@ARGV> contains the command-line arguments intended for
b0c22438 1403the script. C<$#ARGV> is generally the number of arguments minus
1404one, because C<$ARGV[0]> is the first argument, I<not> the program's
1405command name itself. See C<$0> for the command name.
84dabc03 1407=item ARGV
1410The special filehandle that iterates over command-line filenames in
1411C<@ARGV>. Usually written as the null filehandle in the angle operator
1412C<< <> >>. Note that currently C<ARGV> only has its magical effect
1413within the C<< <> >> operator; elsewhere it is just a plain filehandle
1414corresponding to the last file opened by C<< <> >>. In particular,
1415passing C<\*ARGV> as a parameter to a function that expects a filehandle
1416may not cause your function to automatically read the contents of all the
1417files in C<@ARGV>.
b0c22438 1419=item ARGVOUT
1422The special filehandle that points to the currently open output file
1423when doing edit-in-place processing with B<-i>. Useful when you have
1424to do a lot of inserting and don't want to keep modifying C<$_>. See
1425L<perlrun> for the B<-i> switch.
5b442a2a 1427=item Handle->output_field_separator( EXPR )
84dabc03 1428
1431=item $OFS
1433=item $,
1436The output field separator for the print operator. If defined, this
1437value is printed between each of print's arguments. Default is C<undef>.
1439Mnemonic: what is printed when there is a "," in your print statement.
5b442a2a 1441=item HANDLE->input_line_number( EXPR )
b0c22438 1442
1445=item $NR
1447=item $.
1448X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1450Current line number for the last filehandle accessed.
1452Each filehandle in Perl counts the number of lines that have been read
7333b1c4 1453from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1454constitutes a line may not match yours.) When a line is read from a
1455filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1456C<seek()> is called on it, C<$.> becomes an alias to the line counter
1457for that filehandle.
1459You can adjust the counter by assigning to C<$.>, but this will not
1460actually move the seek pointer. I<Localizing C<$.> will not localize
1461the filehandle's line count>. Instead, it will localize perl's notion
1462of which filehandle C<$.> is currently aliased to.
1464C<$.> is reset when the filehandle is closed, but B<not> when an open
1465filehandle is reopened without an intervening C<close()>. For more
1466details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1467an explicit close, line numbers increase across C<ARGV> files (but see
1468examples in L<perlfunc/eof>).
1470You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1471line counter for a given filehandle without having to worry about
1472which handle you last accessed.
1474Mnemonic: many programs use "." to mean the current line number.
5b442a2a 1476=item HANDLE->input_record_separator( EXPR )
b0c22438 1477
1480=item $RS
1482=item $/
84dabc03 1485The input record separator, newline by default. This influences Perl's
7333b1c4 1486idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1487treating empty lines as a terminator if set to the null string (an
1488empty line cannot contain any spaces or tabs). You may set it to a
1489multi-character string to match a multi-character terminator, or to
1490C<undef> to read through the end of file. Setting it to C<"\n\n">
1491means something slightly different than setting to C<"">, if the file
1492contains consecutive empty lines. Setting to C<""> will treat two or
1493more consecutive empty lines as a single empty line. Setting to
1494C<"\n\n"> will blindly assume that the next input character belongs to
1495the next paragraph, even if it's a newline.
b0c22438 1496
1497 local $/; # enable "slurp" mode
1498 local $_ = <FH>; # whole file now here
1499 s/\n[ \t]+/ /g;
7333b1c4 1501Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1502be better for something. :-)
1504Setting C<$/> to a reference to an integer, scalar containing an
1505integer, or scalar that's convertible to an integer will attempt to
1506read records instead of lines, with the maximum record size being the
1507referenced integer. So this:
1509 local $/ = \32768; # or \"32768", or \$var_containing_32768
1510 open my $fh, "<", $myfile or die $!;
1511 local $_ = <$fh>;
fb73857a 1512
7333b1c4 1513will read a record of no more than 32768 bytes from FILE. If you're
b0c22438 1514not reading from a record-oriented file (or your OS doesn't have
1515record-oriented files), then you'll likely get a full chunk of data
7333b1c4 1516with every read. If a record is larger than the record size you've
1517set, you'll get the record back in pieces. Trying to set the record
b0c22438 1518size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1519
b0c22438 1520On VMS, record reads are done with the equivalent of C<sysread>,
1521so it's best not to mix record and non-record reads on the same
5b442a2a 1522file. (This is unlikely to be a problem, because any file you'd
b0c22438 1523want to read in record mode is probably unusable in line mode.)
1524Non-VMS systems do normal I/O, so it's safe to mix record and
1525non-record reads of a file.
5c055ba3 1526
7333b1c4 1527See also L<perlport/"Newlines">. Also see C<$.>.
9bf22702 1528
b0c22438 1529Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1530
5b442a2a 1531=item Handle->output_record_separator( EXPR )
84902520 1532
b0c22438 1533=item $OUTPUT_RECORD_SEPARATOR
84902520 1534
b0c22438 1535=item $ORS
84902520 1536
b0c22438 1537=item $\
84902520 1539
b0c22438 1540The output record separator for the print operator. If defined, this
1541value is printed after the last of print's arguments. Default is C<undef>.
84902520 1542
b0c22438 1543Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1544Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1545
5b442a2a 1546=item HANDLE->autoflush( EXPR )
84dabc03 1550=item $|
1551X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1552
84dabc03 1553If set to nonzero, forces a flush right away and after every write or
7333b1c4 1554print on the currently selected output channel. Default is 0
84dabc03 1555(regardless of whether the channel is really buffered by the system or
1556not; C<$|> tells you only whether you've asked Perl explicitly to
1557flush after each write). STDOUT will typically be line buffered if
5b442a2a 1558output is to the terminal and block buffered otherwise. Setting this
84dabc03 1559variable is useful primarily when you are outputting to a pipe or
1560socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1561want to see the output as it's happening. This has no effect on input
7333b1c4 1562buffering. See L<perlfunc/getc> for that. See L<perldoc/select> on
84dabc03 1563how to select the output channel. See also L<IO::Handle>.
1565Mnemonic: when you want your pipes to be piping hot.
84902520 1568
b0c22438 1569=head3 Variables related to formats
83ee9e09 1570
b0c22438 1571The special variables for formats are a subset of those for
69b55ccc 1572filehandles. See L<perlform> for more information about Perl's
83ee9e09 1574
b0c22438 1575=over 8
83ee9e09 1576
84dabc03 1577=item $ACCUMULATOR
1579=item $^A
1582The current value of the C<write()> accumulator for C<format()> lines.
1583A format contains C<formline()> calls that put their result into
7333b1c4 1584C<$^A>. After calling its format, C<write()> prints out the contents
84dabc03 1585of C<$^A> and empties. So you never really see the contents of C<$^A>
1586unless you call C<formline()> yourself and then look at it. See
1587L<perlform> and L<perlfunc/formline()>.
5b442a2a 1589=item HANDLE->format_formfeed(EXPR)
84dabc03 1593=item $^L
1596What formats output as a form feed. The default is C<\f>.
b0c22438 1598=item HANDLE->format_page_number(EXPR)
83ee9e09 1599
b0c22438 1600=item $FORMAT_PAGE_NUMBER
83ee9e09 1601
b0c22438 1602=item $%
83ee9e09 1604
b0c22438 1605The current page number of the currently selected output channel.
83ee9e09 1606
b0c22438 1607Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1608
b0c22438 1609=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1610
b0c22438 1611=item $FORMAT_LINES_LEFT
66558a10 1612
b0c22438 1613=item $-
fb73857a 1615
b0c22438 1616The number of lines left on the page of the currently selected output
fa05a9fd 1618
b0c22438 1619Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1620
84dabc03 1621=item Handle->format_line_break_characters EXPR
fb73857a 1622
a0d0e21e 1624
84dabc03 1625=item $:
a0d0e21e 1627
84dabc03 1628The current set of characters after which a string may be broken to
1629fill continuation fields (starting with C<^>) in a format. The default is
1630S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1631
84dabc03 1632Mnemonic: a "colon" in poetry is a part of a line.
1634=item HANDLE->format_lines_per_page(EXPR)
1638=item $=
1641The current page length (printable lines) of the currently selected
1642output channel. The default is 60.
1644Mnemonic: = has horizontal lines.
7c36658b 1645
b0c22438 1646=item HANDLE->format_top_name(EXPR)
7c36658b 1647
b0c22438 1648=item $FORMAT_TOP_NAME
a05d7ebb 1649
b0c22438 1650=item $^
fde18df1 1652
b0c22438 1653The name of the current top-of-page format for the currently selected
1654output channel. The default is the name of the filehandle with C<_TOP>
1655appended. For example, the default format top name for the C<STDOUT>
1656filehanlde is C<STDOUT_TOP>.
e07ea26a 1657
b0c22438 1658Mnemonic: points to top of page.
e07ea26a 1659
84dabc03 1660=item HANDLE->format_name(EXPR)
16070b82 1661
84dabc03 1662=item $FORMAT_NAME
aa2f2a36 1663
84dabc03 1664=item $~
1665X<$~> X<$FORMAT_NAME>
aa2f2a36 1666
84dabc03 1667The name of the current report format for the currently selected
1668output channel. The default format name is the same as the filehandle
1669name. For example, the default format name for the C<STDOUT>
1670filehandle is just C<STDOUT>.
16070b82 1671
84dabc03 1672Mnemonic: brother to C<$^>.
16070b82 1673
b0c22438 1674=back
a0d0e21e 1675
84dabc03 1676=head2 Error Variables
b0c22438 1677X<error> X<exception>
a0d0e21e 1678
b0c22438 1679The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1680about different types of error conditions that may appear during
1681execution of a Perl program. The variables are shown ordered by
1682the "distance" between the subsystem which reported the error and
1683the Perl process. They correspond to errors detected by the Perl
1684interpreter, C library, operating system, or an external program,
4438c4b7 1686
b0c22438 1687To illustrate the differences between these variables, consider the
7333b1c4 1688following Perl expression, which uses a single-quoted string. After
1689execution of this statement, perl may have set all four special error
4438c4b7 1691
ca1b95ae 1692 eval q{
7333b1c4 1693 open my $pipe, "/cdrom/install |" or die $!;
1694 my @res = <$pipe>;
1695 close $pipe or die "bad pipe: $?, $!";
1696 };
a0d0e21e 1697
7333b1c4 1698When perl executes the C<eval()> expression, it translates the
1699C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
69b55ccc 1700and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1701the C library's C<errno> if one of these calls fails.
2a8c8378 1702
84dabc03 1703C<$@> is set if the string to be C<eval>-ed did not compile (this may
1704happen if C<open> or C<close> were imported with bad prototypes), or
7333b1c4 1705if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1706value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1707will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1708
84dabc03 1709Under a few operating systems, C<$^E> may contain a more verbose error
1710indicator, such as in this case, "CDROM tray not closed." Systems that
1711do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1712
b0c22438 1713Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1714F</cdrom/install> fails. The upper eight bits reflect specific error
1715conditions encountered by the program (the program's C<exit()> value).
1716The lower eight bits reflect mode of failure, like signal death and
1717core dump information. See C<wait(2)> for details. In contrast to
1718C<$!> and C<$^E>, which are set only if error condition is detected,
1719the variable C<$?> is set on each C<wait> or pipe C<close>,
1720overwriting the old value. This is more like C<$@>, which on every
1721C<eval()> is always set on failure and cleared on success.
a0d0e21e 1722
b0c22438 1723For more details, see the individual descriptions at C<$@>, C<$!>,
1724C<$^E>, and C<$?>.
38e4f4ae 1725
0b9346e6 1726=over 8
b0c22438 1728=item ${^CHILD_ERROR_NATIVE}
a0d0e21e 1730
b0c22438 1731The native status returned by the last pipe close, backtick (C<``>)
1732command, successful call to C<wait()> or C<waitpid()>, or from the
1733C<system()> operator. On POSIX-like systems this value can be decoded
1735WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1736
b0c22438 1737Under VMS this reflects the actual VMS exit status; i.e. it is the
1738same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1739
b0c22438 1740This variable was added in Perl 5.8.9.
a0d0e21e 1741
5b442a2a 1742=item $EXTENDED_OS_ERROR
84dabc03 1744=item $^E
1747Error information specific to the current operating system. At the
1748moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1749for MacPerl). On all other platforms, C<$^E> is always just the same
1750as C<$!>.
1752Under VMS, C<$^E> provides the VMS status value from the last system
1753error. This is more specific information about the last system error
1754than that provided by C<$!>. This is particularly important when C<$!>
1755is set to B<EVMSERR>.
1757Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1758API either via CRT, or directly from perl.
1760Under Win32, C<$^E> always returns the last error information reported
1761by the Win32 call C<GetLastError()> which describes the last error
1762from within the Win32 API. Most Win32-specific code will report errors
1763via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1764portable Perl code will report errors via C<$!>.
1766Caveats mentioned in the description of C<$!> generally apply to
1767C<$^E>, also.
1769This variable was added in Perl 5.003.
1771Mnemonic: Extra error explanation.
0b9346e6 1772
84dabc03 1773=item $EXCEPTIONS_BEING_CAUGHT
1775=item $^S
1778Current state of the interpreter.
ca1b95ae 1780 $^S State
1781 --------- -------------------
1782 undef Parsing module/eval
1783 true (1) Executing an eval
1784 false (0) Otherwise
84dabc03 1785
1786The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1789This variable was added in Perl 5.004.
1791=item $WARNING
1793=item $^W
1794X<$^W> X<$WARNING>
1796The current value of the warning switch, initially true if B<-w> was
1797used, false otherwise, but directly modifiable.
1799See also L<warnings>.
0b9346e6 1801Mnemonic: related to the B<-w> switch.
84dabc03 1802
1803=item ${^WARNING_BITS}
ca1b95ae 1804X<${^WARNING_BITS}>
84dabc03 1805
1806The current set of warning checks enabled by the C<use warnings> pragma.
1807See the documentation of C<warnings> for more details.
1809This variable was added in Perl 5.10.
b0c22438 1811=item $OS_ERROR
5ccee41e 1812
b0c22438 1813=item $ERRNO
5ccee41e 1814
b0c22438 1815=item $!
1816X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1817
b0c22438 1818If used numerically, yields the current value of the C C<errno>
1819variable, or in other words, if a system or library call fails, it
1820sets this variable. This means that the value of C<$!> is meaningful
1821only I<immediately> after a B<failure>:
9b0e6e7a 1822
ca1b95ae 1823 if (open my $fh, "<", $filename) {
1824 # Here $! is meaningless.
1825 ...
1826 }
1827 else {
1828 # ONLY here is $! meaningful.
1829 ...
1830 # Already here $! might be meaningless.
b0c22438 1831 }
1832 # Since here we might have either success or failure,
1833 # here $! is meaningless.
a0d0e21e 1834
7333b1c4 1835The I<meaningless> stands for anything: zero, non-zero,
84dabc03 1836C<undef>. A successful system or library call does B<not> set the
1837variable to zero.
a0d0e21e 1838
84dabc03 1839If used as a string, yields the corresponding system error string. You
1840can assign a number to C<$!> to set I<errno> if, for instance, you
1841want C<"$!"> to return the string for error I<n>, or you want to set
1842the exit value for the C<die()> operator.
d54b56d5 1843
b0c22438 1844Mnemonic: What just went bang?
314d39ce 1845
b0c22438 1846=item %OS_ERROR
fb73857a 1847
b0c22438 1848=item %ERRNO
fb73857a 1849
b0c22438 1850=item %!
5b442a2a 1851X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1852
b0c22438 1853Each element of C<%!> has a true value only if C<$!> is set to that
1854value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1855value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1856such file or directory" (or its moral equivalent: not all operating
1857systems give that exact error, and certainly not all languages). To
1858check if a particular key is meaningful on your system, use C<exists
1859$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1860for more information, and also see L</$!>.
a0d0e21e 1861
b0c22438 1862This variable was added in Perl 5.005.
44f0be63 1863
84dabc03 1864=item $CHILD_ERROR
b687b08b 1865
84dabc03 1866=item $?
1867X<$?> X<$CHILD_ERROR>
a0d0e21e 1868
84dabc03 1869The status returned by the last pipe close, backtick (C<``>) command,
1870successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1871operator. This is just the 16-bit status word returned by the
1872traditional Unix C<wait()> system call (or else is made up to look
1873like it). Thus, the exit value of the subprocess is really (C<<< $? >>
18748 >>>), and C<$? & 127> gives which signal, if any, the process died
1875from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1876
84dabc03 1877Additionally, if the C<h_errno> variable is supported in C, its value
1878is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1879
84dabc03 1880If you have installed a signal handler for C<SIGCHLD>, the
1881value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1882
84dabc03 1883Inside an C<END> subroutine C<$?> contains the value that is going to be
1884given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1885change the exit status of your program. For example:
a0d0e21e 1886
84dabc03 1887 END {
1888 $? = 1 if $? == 255; # die would make it 255
1889 }
a0d0e21e 1890
84dabc03 1891Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1892actual VMS exit status, instead of the default emulation of POSIX
1893status; see L<perlvms/$?> for details.
1895Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1896
b0c22438 1897=item $EVAL_ERROR
f648820c 1898
b0c22438 1899=item $@
1900X<$@> X<$EVAL_ERROR>
a0d0e21e 1901
0b9346e6 1902The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1903the null string, the last C<eval()> parsed and executed correctly
b0c22438 1904(although the operations you invoked may have failed in the normal
a0d0e21e 1906
b0c22438 1907Warning messages are not collected in this variable. You can, however,
1908set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1909described in L</%SIG>.
748a9306 1910
b0c22438 1911Mnemonic: Where was the syntax error "at"?
7f315d2e 1912
b0c22438 1913=back
7f315d2e 1914
b0c22438 1915=head2 Deprecated and removed variables
7f315d2e 1916
0b9346e6 1917Deprecating a variable announces the intent of the perl maintainers to
84dabc03 1918eventually remove the variable from the langauge. It may still be
b0c22438 1919available despite its status. Using a deprecated variable triggers
1920a warning.
7f315d2e 1921
84dabc03 1922Once a variable is removed, its use triggers an error telling you
b0c22438 1923the variable is unsupported.
7f315d2e 1924
84dabc03 1925See L<perldiag> for details about error messages.
7f315d2e 1926
b0c22438 1927=over 8
7f315d2e 1928
5b442a2a 1929=item $OFMT
84dabc03 1931=item $#
5b442a2a 1932X<$#> X<$OFMT>
84dabc03 1933
1934C<$#> was a variable that you could be use to format printed numbers.
1935After a deprecation cycle, its magic was removed in Perl 5.10 and
1936using it now triggers a warning: C<$# is no longer supported>.
1938This is not the sigil you use in front of an array name to get the
1939last index, like C<$#array>. That's still how you get the last index
1940of an array in Perl. The two have nothing to do with each other.
1942Deprecated in Perl 5.
1944Removed in Perl 5.10.
1946=item $*
84dabc03 1949C<$*> was a variable that you could use to enable multiline matching.
1950After a deprecation cycle, its magic was removed in Perl 5.10.
1951Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 1952You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 1953
b0c22438 1954Deprecated in Perl 5.
7f315d2e 1955
b0c22438 1956Removed in Perl 5.10.
7f315d2e 1957
5b442a2a 1958=item $ARRAY_BASE
84dabc03 1960=item $[
5b442a2a 1961X<$[> X<$ARRAY_BASE>
84dabc03 1962
1963This variable stores the index of the first element in an array, and
1964of the first character in a substring. You use to be able to assign to
1965this variable, but you can't do that anymore. It's now always 0, like
0b9346e6 1966it should be.
84dabc03 1967
1968Mnemonic: [ begins subscripts.
1970This variable is read-only.
0b9346e6 1972Deprecated in Perl 5.12.
84dabc03 1973
5b442a2a 1974=item $OLD_PERL_VERSION
b0c22438 1976=item $]
5b442a2a 1977X<$]> X<$OLD_PERL_VERSION>
55602bd2 1978
d4ba9bf2 1979See C<$^V> for a more modern representation of the Perl version that allows
1980accurate string comparisons.
b0c22438 1982The version + patchlevel / 1000 of the Perl interpreter. This variable
1983can be used to determine whether the Perl interpreter executing a
1984script is in the right range of versions:
55602bd2 1985
b0c22438 1986 warn "No checksumming!\n" if $] < 3.019;
55602bd2 1987
d4ba9bf2 1988The floating point representation can sometimes lead to inaccurate
1989numeric comparisons.
b0c22438 1991See also the documentation of C<use VERSION> and C<require VERSION>
1992for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 1993
b0c22438 1994Mnemonic: Is this version of perl in the right bracket?
19799a22 1995
b0c22438 1996Deprecated in Perl 5.6.
19799a22 1997
b0c22438 1998=back
2b92dfce 1999
0b9346e6 2000=cut