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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
10punctuational names have reasonable mnemonics, or analogues in one of
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 $_
50
51The default input and pattern-searching space. The following pairs are
52equivalent:
53
54 while (<>) {...} # only equivalent in while!
55 while ($_ = <>) {...}
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
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109=item $<I<digit>>
110
111Contains the subpattern from the corresponding set of parentheses in
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
130enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted
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()
139enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
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 $*
164
165Set to 1 to do multiline matching within a string, 0 to tell Perl
166that it can assume that strings contain a single line, for the purpose
167of optimizing pattern matches. Pattern matches on strings containing
168multiple newlines can produce confusing results when "C<$*>" is 0. Default
169is 0. (Mnemonic: * matches multiple things.) Note that this variable
170only influences the interpretation of "C<^>" and "C<$>". A literal newline can
171be searched for even when C<$* == 0>.
172
173Use of "C<$*>" is deprecated in Perl 5.
174
175=item input_line_number HANDLE EXPR
176
177=item $INPUT_LINE_NUMBER
178
179=item $NR
180
181=item $.
182
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183The current input line number for the last file handle from
184which you read (or performed a C<seek> or <tell> on). An
185explicit close on a filehandle resets the line number. Since
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186"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187across ARGV files (but see examples under eof()). Localizing C<$.> 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
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201variable, including treating empty lines as delimiters if set to the
202null string. (Note: An empty line can not contain any spaces or
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 $|
221
222If set to nonzero, forces a flush after every write or print on the
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223currently selected output channel. Default is 0 (regardless of whether
224the channel is actually buffered by the system or not; C<$|> only tells
225you whether you've asked Perl to explicitly flush after each write).
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
242print operator simply prints out the comma separated fields you
243specify. In order to get behavior more like B<awk>, set this variable
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 $\
255
256The output record separator for the print operator. Ordinarily the
257print operator simply prints out the comma separated fields you
258specify, with no trailing newline or record separator assumed. In
259order to get behavior more like B<awk>, set this variable as you would
260set B<awk>'s ORS variable to specify what is printed at the end of the
261print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
262print. Also, it's just like /, but it's what you get "back" from
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
302Consider using "real" multi-dimensional arrays in Perl 5.
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.)
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315
316Use of "C<$#>" is deprecated in Perl 5.
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
382What formats output to perform a formfeed. Default is \f.
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 $?
398
399The status returned by the last pipe close, backtick (C<``>) command,
400or system() operator. Note that this is the status word returned by
401the wait() system call, so the exit value of the subprocess is actually
402(C<$? 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
406=item $OS_ERROR
407
408=item $ERRNO
409
410=item $!
411
412If used in a numeric context, yields the current value of errno, with
413all the usual caveats. (This means that you shouldn't depend on the
414value of "C<$!>" to be anything in particular unless you've gotten a
415specific error return indicating a system error.) If used in a string
416context, yields the corresponding system error string. You can assign
417to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the
418string for error I<n>, or you want to set the exit value for the die()
419operator. (Mnemonic: What just went bang?)
420
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421=item $EXTENDED_OS_ERROR
422
423=item $^E
424
425More specific information about the last system error than that
426provided by C<$!>, if available. (If not, it's just C<$!> again.)
427At the moment, this differs from C<$!> only under VMS, where it
428provides the VMS status value from the last system error. The
429caveats mentioned in the description of C<$!> apply here, too.
430(Mnemonic: Extra error explanation.)
431
432
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433=item $EVAL_ERROR
434
435=item $@
436
437The Perl syntax error message from the last eval() command. If null, the
438last eval() parsed and executed correctly (although the operations you
439invoked may have failed in the normal fashion). (Mnemonic: Where was
440the syntax error "at"?)
441
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442Note that warning messages are not collected in this variable. You can,
443however, set up a routine to process warnings by setting $SIG{__WARN__} below.
444
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445=item $PROCESS_ID
446
447=item $PID
448
449=item $$
450
451The process number of the Perl running this script. (Mnemonic: same
452as shells.)
453
454=item $REAL_USER_ID
455
456=item $UID
457
458=item $<
459
460The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
461if you're running setuid.)
462
463=item $EFFECTIVE_USER_ID
464
465=item $EUID
466
467=item $>
468
469The effective uid of this process. Example:
470
471 $< = $>; # set real to effective uid
472 ($<,$>) = ($>,$<); # swap real and effective uid
473
474(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
475"C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid().
476
477=item $REAL_GROUP_ID
478
479=item $GID
480
481=item $(
482
483The real gid of this process. If you are on a machine that supports
484membership in multiple groups simultaneously, gives a space separated
485list of groups you are in. The first number is the one returned by
486getgid(), and the subsequent ones by getgroups(), one of which may be
487the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
488things. The real gid is the group you I<LEFT>, if you're running setgid.)
489
490=item $EFFECTIVE_GROUP_ID
491
492=item $EGID
493
494=item $)
495
496The effective gid of this process. If you are on a machine that
497supports membership in multiple groups simultaneously, gives a space
498separated list of groups you are in. The first number is the one
499returned by getegid(), and the subsequent ones by getgroups(), one of
500which may be the same as the first number. (Mnemonic: parentheses are
501used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
502you, if you're running setgid.)
503
504Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines
505that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>"
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506can only be swapped on machines supporting setregid(). Because Perl doesn't
507currently use initgroups(), you can't set your group vector to multiple groups.
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508
509=item $PROGRAM_NAME
510
511=item $0
512
513Contains the name of the file containing the Perl script being
514executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
515program sees. This is more useful as a way of indicating the
516current program state than it is for hiding the program you're running.
517(Mnemonic: same as B<sh> and B<ksh>.)
518
519=item $[
520
521The index of the first element in an array, and of the first character
522in a substring. Default is 0, but you could set it to 1 to make
523Perl behave more like B<awk> (or Fortran) when subscripting and when
524evaluating the index() and substr() functions. (Mnemonic: [ begins
525subscripts.)
526
527As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
528and cannot influence the behavior of any other file. Its use is
529discouraged.
530
531=item $PERL_VERSION
532
533=item $]
534
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535The string printed out when you say C<perl -v>.
536(This is currently I<BROKEN>).
537It can be used to
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538determine at the beginning of a script whether the perl interpreter
539executing the script is in the right range of versions. If used in a
540numeric context, returns the version + patchlevel / 1000. Example:
541
542 # see if getc is available
543 ($version,$patchlevel) =
544 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
545 print STDERR "(No filename completion available.)\n"
546 if $version * 1000 + $patchlevel < 2016;
547
548or, used numerically,
549
550 warn "No checksumming!\n" if $] < 3.019;
551
552(Mnemonic: Is this version of perl in the right bracket?)
553
554=item $DEBUGGING
555
556=item $^D
557
558The current value of the debugging flags. (Mnemonic: value of B<-D>
559switch.)
560
561=item $SYSTEM_FD_MAX
562
563=item $^F
564
565The maximum system file descriptor, ordinarily 2. System file
566descriptors are passed to exec()ed processes, while higher file
567descriptors are not. Also, during an open(), system file descriptors are
568preserved even if the open() fails. (Ordinary file descriptors are
569closed before the open() is attempted.) Note that the close-on-exec
570status of a file descriptor will be decided according to the value of
571C<$^F> at the time of the open, not the time of the exec.
572
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573=item $^H
574
575The current set of syntax checks enabled by C<use strict>. See the
576documentation of C<strict> for more details.
577
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578=item $INPLACE_EDIT
579
580=item $^I
581
582The current value of the inplace-edit extension. Use C<undef> to disable
583inplace editing. (Mnemonic: value of B<-i> switch.)
584
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6e2995f4 586
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587=item $^O
588
589The name of the operating system under which this copy of Perl was
590built, as determined during the configuration process. The value
591is identical to C<$Config{'osname'}>.
592
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593=item $PERLDB
594
595=item $^P
596
597The internal flag that the debugger clears so that it doesn't debug
5c055ba3 598itself. You could conceivably disable debugging yourself by clearing
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599it.
600
601=item $BASETIME
602
603=item $^T
604
605The time at which the script began running, in seconds since the
606epoch (beginning of 1970). The values returned by the B<-M>, B<-A>
607and B<-C> filetests are
608based on this value.
609
610=item $WARNING
611
612=item $^W
613
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614The current value of the warning switch, either TRUE or FALSE.
615(Mnemonic: related to the B<-w> switch.)
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616
617=item $EXECUTABLE_NAME
618
619=item $^X
620
621The name that the Perl binary itself was executed as, from C's C<argv[0]>.
622
623=item $ARGV
624
625contains the name of the current file when reading from <>.
626
627=item @ARGV
628
629The array @ARGV contains the command line arguments intended for the
630script. Note that C<$#ARGV> is the generally number of arguments minus
631one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See
632"C<$0>" for the command name.
633
634=item @INC
635
636The array @INC contains the list of places to look for Perl scripts to
637be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
638initially consists of the arguments to any B<-I> command line switches,
6e2995f4 639followed by the default Perl library, probably F</usr/local/lib/perl>,
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640followed by ".", to represent the current directory. If you need to
641modify this at runtime, you should use the C<use lib> pragma in order
642to also get the machine-dependent library properly loaded:
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644 use lib '/mypath/libdir/';
645 use SomeMod;
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647=item %INC
648
649The hash %INC contains entries for each filename that has
650been included via C<do> or C<require>. The key is the filename you
651specified, and the value is the location of the file actually found.
652The C<require> command uses this array to determine whether a given file
653has already been included.
654
655=item $ENV{expr}
656
657The hash %ENV contains your current environment. Setting a
658value in C<ENV> changes the environment for child processes.
659
660=item $SIG{expr}
661
662The hash %SIG is used to set signal handlers for various
663signals. Example:
664
665 sub handler { # 1st argument is signal name
666 local($sig) = @_;
667 print "Caught a SIG$sig--shutting down\n";
668 close(LOG);
669 exit(0);
670 }
671
672 $SIG{'INT'} = 'handler';
673 $SIG{'QUIT'} = 'handler';
674 ...
675 $SIG{'INT'} = 'DEFAULT'; # restore default action
676 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
677
678The %SIG array only contains values for the signals actually set within
679the Perl script. Here are some other examples:
680
681 $SIG{PIPE} = Plumber; # SCARY!!
682 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
683 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
684 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
685
686The one marked scary is problematic because it's a bareword, which means
687sometimes it's a string representing the function, and sometimes it's
688going to call the subroutine call right then and there! Best to be sure
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689and quote it or take a reference to it. *Plumber works too. See L<perlsubs>.
690
691Certain internal hooks can be also set using the %SIG hash. The
692routine indicated by $SIG{__WARN__} is called when a warning message is
693about to be printed. The warning message is passed as the first
694argument. The presence of a __WARN__ hook causes the ordinary printing
695of warnings to STDERR to be suppressed. You can use this to save warnings
696in a variable, or turn warnings into fatal errors, like this:
697
698 local $SIG{__WARN__} = sub { die $_[0] };
699 eval $proggie;
700
701The routine indicated by $SIG{__DIE__} is called when a fatal exception
702is about to be thrown. The error message is passed as the first
703argument. When a __DIE__ hook routine returns, the exception
704processing continues as it would have in the absence of the hook,
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705unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
706The __DIE__ handler is explicitly disabled during the call, so that you
707can die from a __DIE__ handler. Similarly for __WARN__.
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708
709=back