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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
9The following names have special meaning to Perl. Most of the
5f05dabc 10punctuation names have reasonable mnemonics, or analogues in one of
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11the shells. Nevertheless, if you wish to use the long variable names,
12you just need to say
13
14 use English;
15
16at the top of your program. This will alias all the short names to the
17long names in the current package. Some of them even have medium names,
18generally borrowed from B<awk>.
19
20To go a step further, those variables that depend on the currently
21selected filehandle may instead be set by calling an object method on
22the FileHandle object. (Summary lines below for this contain the word
23HANDLE.) First you must say
24
25 use FileHandle;
26
27after which you may use either
28
29 method HANDLE EXPR
30
31or
32
33 HANDLE->method(EXPR)
34
35Each of the methods returns the old value of the FileHandle attribute.
36The methods each take an optional EXPR, which if supplied specifies the
37new value for the FileHandle attribute in question. If not supplied,
38most of the methods do nothing to the current value, except for
39autoflush(), which will assume a 1 for you, just to be different.
40
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41A few of these variables are considered "read-only". This means that if
42you try to assign to this variable, either directly or indirectly through
43a reference, you'll raise a run-time exception.
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44
45=over 8
46
47=item $ARG
48
49=item $_
50
51The default input and pattern-searching space. The following pairs are
52equivalent:
53
5f05dabc 54 while (<>) {...} # equivalent in only while!
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55 while ($_ = <>) {...}
56
57 /^Subject:/
58 $_ =~ /^Subject:/
59
60 tr/a-z/A-Z/
61 $_ =~ tr/a-z/A-Z/
62
63 chop
64 chop($_)
65
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66Here are the places where Perl will assume $_ even if you
67don't use it:
68
69=over 3
70
71=item *
72
73Various unary functions, including functions like ord() and int(), as well
74as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
75STDIN.
76
77=item *
78
79Various list functions like print() and unlink().
80
81=item *
82
83The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84without an C<=~> operator.
85
86=item *
87
88The default iterator variable in a C<foreach> loop if no other
89variable is supplied.
90
91=item *
92
93The implicit iterator variable in the grep() and map() functions.
94
95=item *
96
97The default place to put an input record when a C<E<lt>FHE<gt>>
98operation's result is tested by itself as the sole criterion of a C<while>
99test. Note that outside of a C<while> test, this will not happen.
100
101=back
102
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103(Mnemonic: underline is understood in certain operations.)
104
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105=back
106
107=over 8
108
a8f8344d 109=item $E<lt>I<digit>E<gt>
a0d0e21e 110
5f05dabc 111Contains the sub-pattern from the corresponding set of parentheses in
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112the last pattern matched, not counting patterns matched in nested
113blocks that have been exited already. (Mnemonic: like \digit.)
114These variables are all read-only.
115
116=item $MATCH
117
118=item $&
119
120The string matched by the last successful pattern match (not counting
121any matches hidden within a BLOCK or eval() enclosed by the current
122BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
123
124=item $PREMATCH
125
126=item $`
127
128The string preceding whatever was matched by the last successful
129pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 130enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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131string.) This variable is read-only.
132
133=item $POSTMATCH
134
135=item $'
136
137The string following whatever was matched by the last successful
138pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 139enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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140string.) Example:
141
142 $_ = 'abcdefghi';
143 /def/;
144 print "$`:$&:$'\n"; # prints abc:def:ghi
145
146This variable is read-only.
147
148=item $LAST_PAREN_MATCH
149
150=item $+
151
152The last bracket matched by the last search pattern. This is useful if
153you don't know which of a set of alternative patterns matched. For
154example:
155
156 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
157
158(Mnemonic: be positive and forward looking.)
159This variable is read-only.
160
161=item $MULTILINE_MATCHING
162
163=item $*
164
5f05dabc 165Set to 1 to do multi-line matching within a string, 0 to tell Perl
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166that it can assume that strings contain a single line, for the purpose
167of optimizing pattern matches. Pattern matches on strings containing
168multiple newlines can produce confusing results when "C<$*>" is 0. Default
169is 0. (Mnemonic: * matches multiple things.) Note that this variable
5f05dabc 170influences the interpretation of only "C<^>" and "C<$>". A literal newline can
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171be searched for even when C<$* == 0>.
172
5f05dabc 173Use of "C<$*>" is deprecated in modern perls.
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174
175=item input_line_number HANDLE EXPR
176
177=item $INPUT_LINE_NUMBER
178
179=item $NR
180
181=item $.
182
6e2995f4 183The current input line number for the last file handle from
a8f8344d 184which you read (or performed a C<seek> or C<tell> on). An
5f05dabc 185explicit close on a filehandle resets the line number. Because
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186"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187across ARGV files (but see examples under eof()). Localizing C<$.> has
188the effect of also localizing Perl's notion of "the last read
189filehandle". (Mnemonic: many programs use "." to mean the current line
190number.)
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191
192=item input_record_separator HANDLE EXPR
193
194=item $INPUT_RECORD_SEPARATOR
195
196=item $RS
197
198=item $/
199
200The input record separator, newline by default. Works like B<awk>'s RS
303f2f76 201variable, including treating empty lines as delimiters if set to the
a8f8344d 202null string. (Note: An empty line cannot contain any spaces or
303f2f76 203tabs.) You may set it to a multicharacter string to match a
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204multi-character delimiter. Note that setting it to C<"\n\n"> means
205something slightly different than setting it to C<"">, if the file
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206contains consecutive empty lines. Setting it to C<""> will treat two
207or more consecutive empty lines as a single empty line. Setting it to
208C<"\n\n"> will blindly assume that the next input character belongs to
209the next paragraph, even if it's a newline. (Mnemonic: / is used to
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210delimit line boundaries when quoting poetry.)
211
212 undef $/;
213 $_ = <FH>; # whole file now here
214 s/\n[ \t]+/ /g;
215
216=item autoflush HANDLE EXPR
217
218=item $OUTPUT_AUTOFLUSH
219
220=item $|
221
222If set to nonzero, forces a flush after every write or print on the
6e2995f4 223currently selected output channel. Default is 0 (regardless of whether
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224the channel is actually buffered by the system or not; C<$|> tells you
225only whether you've asked Perl explicitly to flush after each write).
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226Note that STDOUT will typically be line buffered if output is to the
227terminal and block buffered otherwise. Setting this variable is useful
228primarily when you are outputting to a pipe, such as when you are running
229a Perl script under rsh and want to see the output as it's happening. This
230has no effect on input buffering.
cb1a09d0 231(Mnemonic: when you want your pipes to be piping hot.)
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232
233=item output_field_separator HANDLE EXPR
234
235=item $OUTPUT_FIELD_SEPARATOR
236
237=item $OFS
238
239=item $,
240
241The output field separator for the print operator. Ordinarily the
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242print operator simply prints out the comma-separated fields you
243specify. To get behavior more like B<awk>, set this variable
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244as you would set B<awk>'s OFS variable to specify what is printed
245between fields. (Mnemonic: what is printed when there is a , in your
246print statement.)
247
248=item output_record_separator HANDLE EXPR
249
250=item $OUTPUT_RECORD_SEPARATOR
251
252=item $ORS
253
254=item $\
255
256The output record separator for the print operator. Ordinarily the
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257print operator simply prints out the comma-separated fields you
258specify, with no trailing newline or record separator assumed.
259To get behavior more like B<awk>, set this variable as you would
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260set B<awk>'s ORS variable to specify what is printed at the end of the
261print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
a8f8344d 262print. Also, it's just like C<$/>, but it's what you get "back" from
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263Perl.)
264
265=item $LIST_SEPARATOR
266
267=item $"
268
269This is like "C<$,>" except that it applies to array values interpolated
270into a double-quoted string (or similar interpreted string). Default
271is a space. (Mnemonic: obvious, I think.)
272
273=item $SUBSCRIPT_SEPARATOR
274
275=item $SUBSEP
276
277=item $;
278
279The subscript separator for multi-dimensional array emulation. If you
280refer to a hash element as
281
282 $foo{$a,$b,$c}
283
284it really means
285
286 $foo{join($;, $a, $b, $c)}
287
288But don't put
289
290 @foo{$a,$b,$c} # a slice--note the @
291
292which means
293
294 ($foo{$a},$foo{$b},$foo{$c})
295
296Default is "\034", the same as SUBSEP in B<awk>. Note that if your
297keys contain binary data there might not be any safe value for "C<$;>".
298(Mnemonic: comma (the syntactic subscript separator) is a
299semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
300taken for something more important.)
301
5f05dabc 302Consider using "real" multi-dimensional arrays.
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303
304=item $OFMT
305
306=item $#
307
308The output format for printed numbers. This variable is a half-hearted
309attempt to emulate B<awk>'s OFMT variable. There are times, however,
310when B<awk> and Perl have differing notions of what is in fact
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311numeric. The initial value is %.I<n>g, where I<n> is the value
312of the macro DBL_DIG from your system's F<float.h>. This is different from
313B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
314explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 315
5f05dabc 316Use of "C<$#>" is deprecated.
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317
318=item format_page_number HANDLE EXPR
319
320=item $FORMAT_PAGE_NUMBER
321
322=item $%
323
324The current page number of the currently selected output channel.
325(Mnemonic: % is page number in B<nroff>.)
326
327=item format_lines_per_page HANDLE EXPR
328
329=item $FORMAT_LINES_PER_PAGE
330
331=item $=
332
333The current page length (printable lines) of the currently selected
334output channel. Default is 60. (Mnemonic: = has horizontal lines.)
335
336=item format_lines_left HANDLE EXPR
337
338=item $FORMAT_LINES_LEFT
339
340=item $-
341
342The number of lines left on the page of the currently selected output
343channel. (Mnemonic: lines_on_page - lines_printed.)
344
345=item format_name HANDLE EXPR
346
347=item $FORMAT_NAME
348
349=item $~
350
351The name of the current report format for the currently selected output
352channel. Default is name of the filehandle. (Mnemonic: brother to
353"C<$^>".)
354
355=item format_top_name HANDLE EXPR
356
357=item $FORMAT_TOP_NAME
358
359=item $^
360
361The name of the current top-of-page format for the currently selected
362output channel. Default is name of the filehandle with _TOP
363appended. (Mnemonic: points to top of page.)
364
365=item format_line_break_characters HANDLE EXPR
366
367=item $FORMAT_LINE_BREAK_CHARACTERS
368
369=item $:
370
371The current set of characters after which a string may be broken to
372fill continuation fields (starting with ^) in a format. Default is
373S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
374poetry is a part of a line.)
375
376=item format_formfeed HANDLE EXPR
377
378=item $FORMAT_FORMFEED
379
380=item $^L
381
5f05dabc 382What formats output to perform a form feed. Default is \f.
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383
384=item $ACCUMULATOR
385
386=item $^A
387
388The current value of the write() accumulator for format() lines. A format
389contains formline() commands that put their result into C<$^A>. After
390calling its format, write() prints out the contents of C<$^A> and empties.
391So you never actually see the contents of C<$^A> unless you call
392formline() yourself and then look at it. See L<perlform> and
393L<perlfunc/formline()>.
394
395=item $CHILD_ERROR
396
397=item $?
398
5f05dabc 399The status returned by the last pipe close, back-tick (C<``>) command,
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400or system() operator. Note that this is the status word returned by
401the wait() system call, so the exit value of the subprocess is actually
402(C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal,
403if any, the process died from, and whether there was a core dump.
404(Mnemonic: similar to B<sh> and B<ksh>.)
405
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406Inside an C<END> subroutine C<$?> contains the value that is going to be
407given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
408change the exit status of the script.
409
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410=item $OS_ERROR
411
412=item $ERRNO
413
414=item $!
415
416If used in a numeric context, yields the current value of errno, with
417all the usual caveats. (This means that you shouldn't depend on the
418value of "C<$!>" to be anything in particular unless you've gotten a
419specific error return indicating a system error.) If used in a string
420context, yields the corresponding system error string. You can assign
5f05dabc 421to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
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422string for error I<n>, or you want to set the exit value for the die()
423operator. (Mnemonic: What just went bang?)
424
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425=item $EXTENDED_OS_ERROR
426
427=item $^E
428
429More specific information about the last system error than that
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430provided by C<$!>, if available. (If not, it's just C<$!> again, except under
431OS/2.)
5f05dabc 432At the moment, this differs from C<$!> under only VMS and OS/2, where it
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433provides the VMS status value from the last system error, and OS/2 error
434code of the last call to OS/2 API which was not directed via CRT. The
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435caveats mentioned in the description of C<$!> apply here, too.
436(Mnemonic: Extra error explanation.)
437
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438Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
439OS/2-specific call is performed, you may need to check both.
5c055ba3 440
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441=item $EVAL_ERROR
442
443=item $@
444
445The Perl syntax error message from the last eval() command. If null, the
446last eval() parsed and executed correctly (although the operations you
447invoked may have failed in the normal fashion). (Mnemonic: Where was
448the syntax error "at"?)
449
748a9306 450Note that warning messages are not collected in this variable. You can,
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451however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
452below.
748a9306 453
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454=item $PROCESS_ID
455
456=item $PID
457
458=item $$
459
460The process number of the Perl running this script. (Mnemonic: same
461as shells.)
462
463=item $REAL_USER_ID
464
465=item $UID
466
467=item $<
468
469The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
470if you're running setuid.)
471
472=item $EFFECTIVE_USER_ID
473
474=item $EUID
475
476=item $>
477
478The effective uid of this process. Example:
479
480 $< = $>; # set real to effective uid
481 ($<,$>) = ($>,$<); # swap real and effective uid
482
483(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
5f05dabc 484"C<$E<lt>>" and "C<$E<gt>>" can be swapped on only machines supporting setreuid().
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485
486=item $REAL_GROUP_ID
487
488=item $GID
489
490=item $(
491
492The real gid of this process. If you are on a machine that supports
493membership in multiple groups simultaneously, gives a space separated
494list of groups you are in. The first number is the one returned by
495getgid(), and the subsequent ones by getgroups(), one of which may be
496the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
497things. The real gid is the group you I<LEFT>, if you're running setgid.)
498
499=item $EFFECTIVE_GROUP_ID
500
501=item $EGID
502
503=item $)
504
505The effective gid of this process. If you are on a machine that
506supports membership in multiple groups simultaneously, gives a space
507separated list of groups you are in. The first number is the one
508returned by getegid(), and the subsequent ones by getgroups(), one of
509which may be the same as the first number. (Mnemonic: parentheses are
510used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
511you, if you're running setgid.)
512
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513Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
514machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
515and "C<$)>" can be swapped on only machines supporting setregid(). Because
516Perl doesn't currently use initgroups(), you can't set your group vector to
517multiple groups.
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518
519=item $PROGRAM_NAME
520
521=item $0
522
523Contains the name of the file containing the Perl script being
524executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
525program sees. This is more useful as a way of indicating the
526current program state than it is for hiding the program you're running.
527(Mnemonic: same as B<sh> and B<ksh>.)
528
529=item $[
530
531The index of the first element in an array, and of the first character
532in a substring. Default is 0, but you could set it to 1 to make
533Perl behave more like B<awk> (or Fortran) when subscripting and when
534evaluating the index() and substr() functions. (Mnemonic: [ begins
535subscripts.)
536
537As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
538and cannot influence the behavior of any other file. Its use is
539discouraged.
540
541=item $PERL_VERSION
542
543=item $]
544
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545The string printed out when you say C<perl -v>.
546(This is currently I<BROKEN>).
547It can be used to
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548determine at the beginning of a script whether the perl interpreter
549executing the script is in the right range of versions. If used in a
550numeric context, returns the version + patchlevel / 1000. Example:
551
552 # see if getc is available
553 ($version,$patchlevel) =
554 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
555 print STDERR "(No filename completion available.)\n"
556 if $version * 1000 + $patchlevel < 2016;
557
558or, used numerically,
559
560 warn "No checksumming!\n" if $] < 3.019;
561
562(Mnemonic: Is this version of perl in the right bracket?)
563
564=item $DEBUGGING
565
566=item $^D
567
568The current value of the debugging flags. (Mnemonic: value of B<-D>
569switch.)
570
571=item $SYSTEM_FD_MAX
572
573=item $^F
574
575The maximum system file descriptor, ordinarily 2. System file
576descriptors are passed to exec()ed processes, while higher file
577descriptors are not. Also, during an open(), system file descriptors are
578preserved even if the open() fails. (Ordinary file descriptors are
579closed before the open() is attempted.) Note that the close-on-exec
580status of a file descriptor will be decided according to the value of
581C<$^F> at the time of the open, not the time of the exec.
582
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583=item $^H
584
585The current set of syntax checks enabled by C<use strict>. See the
586documentation of C<strict> for more details.
587
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588=item $INPLACE_EDIT
589
590=item $^I
591
592The current value of the inplace-edit extension. Use C<undef> to disable
593inplace editing. (Mnemonic: value of B<-i> switch.)
594
5c055ba3 595=item $OSNAME
6e2995f4 596
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597=item $^O
598
599The name of the operating system under which this copy of Perl was
600built, as determined during the configuration process. The value
601is identical to C<$Config{'osname'}>.
602
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603=item $PERLDB
604
605=item $^P
606
607The internal flag that the debugger clears so that it doesn't debug
5c055ba3 608itself. You could conceivably disable debugging yourself by clearing
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609it.
610
611=item $BASETIME
612
613=item $^T
614
615The time at which the script began running, in seconds since the
5f05dabc 616epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
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617and B<-C> filetests are
618based on this value.
619
620=item $WARNING
621
622=item $^W
623
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624The current value of the warning switch, either TRUE or FALSE.
625(Mnemonic: related to the B<-w> switch.)
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626
627=item $EXECUTABLE_NAME
628
629=item $^X
630
631The name that the Perl binary itself was executed as, from C's C<argv[0]>.
632
633=item $ARGV
634
a8f8344d 635contains the name of the current file when reading from E<lt>E<gt>.
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636
637=item @ARGV
638
639The array @ARGV contains the command line arguments intended for the
640script. Note that C<$#ARGV> is the generally number of arguments minus
5f05dabc 641one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
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642"C<$0>" for the command name.
643
644=item @INC
645
646The array @INC contains the list of places to look for Perl scripts to
647be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
648initially consists of the arguments to any B<-I> command line switches,
6e2995f4 649followed by the default Perl library, probably F</usr/local/lib/perl>,
cb1a09d0 650followed by ".", to represent the current directory. If you need to
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651modify this at runtime, you should use the C<use lib> pragma
652to get the machine-dependent library properly loaded also:
a0d0e21e 653
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654 use lib '/mypath/libdir/';
655 use SomeMod;
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657=item %INC
658
659The hash %INC contains entries for each filename that has
660been included via C<do> or C<require>. The key is the filename you
661specified, and the value is the location of the file actually found.
662The C<require> command uses this array to determine whether a given file
663has already been included.
664
665=item $ENV{expr}
666
667The hash %ENV contains your current environment. Setting a
668value in C<ENV> changes the environment for child processes.
669
670=item $SIG{expr}
671
672The hash %SIG is used to set signal handlers for various
673signals. Example:
674
675 sub handler { # 1st argument is signal name
676 local($sig) = @_;
677 print "Caught a SIG$sig--shutting down\n";
678 close(LOG);
679 exit(0);
680 }
681
682 $SIG{'INT'} = 'handler';
683 $SIG{'QUIT'} = 'handler';
684 ...
685 $SIG{'INT'} = 'DEFAULT'; # restore default action
686 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
687
5f05dabc 688The %SIG array contains values for only the signals actually set within
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689the Perl script. Here are some other examples:
690
691 $SIG{PIPE} = Plumber; # SCARY!!
692 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
693 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
694 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
695
696The one marked scary is problematic because it's a bareword, which means
697sometimes it's a string representing the function, and sometimes it's
698going to call the subroutine call right then and there! Best to be sure
a8f8344d 699and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
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700
701Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 702routine indicated by C<$SIG{__WARN__}> is called when a warning message is
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703about to be printed. The warning message is passed as the first
704argument. The presence of a __WARN__ hook causes the ordinary printing
705of warnings to STDERR to be suppressed. You can use this to save warnings
706in a variable, or turn warnings into fatal errors, like this:
707
708 local $SIG{__WARN__} = sub { die $_[0] };
709 eval $proggie;
710
a8f8344d 711The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
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712is about to be thrown. The error message is passed as the first
713argument. When a __DIE__ hook routine returns, the exception
714processing continues as it would have in the absence of the hook,
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715unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
716The __DIE__ handler is explicitly disabled during the call, so that you
717can die from a __DIE__ handler. Similarly for __WARN__.
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718
719=back