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add perltrap entry about "${#a}", as suggested by
[perl5.git] / pod / perltrap.pod
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1=head1 NAME
2
3perltrap - Perl traps for the unwary
4
5=head1 DESCRIPTION
6
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7The biggest trap of all is forgetting to use the B<-w> switch; see
8L<perlrun>. The second biggest trap is not making your entire program
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9runnable under C<use strict>. The third biggest trap is not reading
10the list of changes in this version of Perl; see L<perldelta>.
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11
12=head2 Awk Traps
13
14Accustomed B<awk> users should take special note of the following:
15
16=over 4
17
18=item *
19
20The English module, loaded via
21
22 use English;
23
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24allows you to refer to special variables (like C<$/>) with names (like
25C<$RS>), as though they were in B<awk>; see L<perlvar> for details.
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26
27=item *
28
29Semicolons are required after all simple statements in Perl (except
30at the end of a block). Newline is not a statement delimiter.
31
32=item *
33
34Curly brackets are required on C<if>s and C<while>s.
35
36=item *
37
5db417f7 38Variables begin with "$", "@" or "%" in Perl.
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39
40=item *
41
42Arrays index from 0. Likewise string positions in substr() and
43index().
44
45=item *
46
47You have to decide whether your array has numeric or string indices.
48
49=item *
50
aa689395 51Hash values do not spring into existence upon mere reference.
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52
53=item *
54
55You have to decide whether you want to use string or numeric
56comparisons.
57
58=item *
59
60Reading an input line does not split it for you. You get to split it
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61to an array yourself. And the split() operator has different
62arguments than B<awk>'s.
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63
64=item *
65
66The current input line is normally in $_, not $0. It generally does
67not have the newline stripped. ($0 is the name of the program
68executed.) See L<perlvar>.
69
70=item *
71
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72$E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
73by the last match pattern.
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74
75=item *
76
77The print() statement does not add field and record separators unless
8b0a4b75 78you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
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79the English module.
80
81=item *
82
83You must open your files before you print to them.
84
85=item *
86
87The range operator is "..", not comma. The comma operator works as in
88C.
89
90=item *
91
92The match operator is "=~", not "~". ("~" is the one's complement
93operator, as in C.)
94
95=item *
96
97The exponentiation operator is "**", not "^". "^" is the XOR
98operator, as in C. (You know, one could get the feeling that B<awk> is
99basically incompatible with C.)
100
101=item *
102
103The concatenation operator is ".", not the null string. (Using the
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104null string would render C</pat/ /pat/> unparsable, because the third slash
105would be interpreted as a division operator--the tokenizer is in fact
8b0a4b75 106slightly context sensitive for operators like "/", "?", and "E<gt>".
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107And in fact, "." itself can be the beginning of a number.)
108
109=item *
110
111The C<next>, C<exit>, and C<continue> keywords work differently.
112
113=item *
114
115
116The following variables work differently:
117
118 Awk Perl
119 ARGC $#ARGV or scalar @ARGV
120 ARGV[0] $0
121 FILENAME $ARGV
122 FNR $. - something
123 FS (whatever you like)
124 NF $#Fld, or some such
125 NR $.
126 OFMT $#
127 OFS $,
128 ORS $\
129 RLENGTH length($&)
130 RS $/
131 RSTART length($`)
132 SUBSEP $;
133
134=item *
135
136You cannot set $RS to a pattern, only a string.
137
138=item *
139
140When in doubt, run the B<awk> construct through B<a2p> and see what it
141gives you.
142
143=back
144
145=head2 C Traps
146
147Cerebral C programmers should take note of the following:
148
149=over 4
150
151=item *
152
153Curly brackets are required on C<if>'s and C<while>'s.
154
155=item *
156
157You must use C<elsif> rather than C<else if>.
158
159=item *
160
54310121 161The C<break> and C<continue> keywords from C become in
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162Perl C<last> and C<next>, respectively.
163Unlike in C, these do I<NOT> work within a C<do { } while> construct.
164
165=item *
166
167There's no switch statement. (But it's easy to build one on the fly.)
168
169=item *
170
5db417f7 171Variables begin with "$", "@" or "%" in Perl.
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172
173=item *
174
6dbacca0 175C<printf()> does not implement the "*" format for interpolating
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176field widths, but it's trivial to use interpolation of double-quoted
177strings to achieve the same effect.
178
179=item *
180
181Comments begin with "#", not "/*".
182
183=item *
184
185You can't take the address of anything, although a similar operator
5f05dabc 186in Perl is the backslash, which creates a reference.
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187
188=item *
189
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190C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
191ends up in C<$0>.
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192
193=item *
194
195System calls such as link(), unlink(), rename(), etc. return nonzero for
196success, not 0.
197
198=item *
199
200Signal handlers deal with signal names, not numbers. Use C<kill -l>
201to find their names on your system.
202
203=back
204
205=head2 Sed Traps
206
207Seasoned B<sed> programmers should take note of the following:
208
209=over 4
210
211=item *
212
213Backreferences in substitutions use "$" rather than "\".
214
215=item *
216
217The pattern matching metacharacters "(", ")", and "|" do not have backslashes
218in front.
219
220=item *
221
222The range operator is C<...>, rather than comma.
223
224=back
225
226=head2 Shell Traps
227
228Sharp shell programmers should take note of the following:
229
230=over 4
231
232=item *
233
54310121 234The backtick operator does variable interpolation without regard to
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235the presence of single quotes in the command.
236
237=item *
238
54310121 239The backtick operator does no translation of the return value, unlike B<csh>.
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240
241=item *
242
243Shells (especially B<csh>) do several levels of substitution on each
5f05dabc 244command line. Perl does substitution in only certain constructs
54310121 245such as double quotes, backticks, angle brackets, and search patterns.
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246
247=item *
248
249Shells interpret scripts a little bit at a time. Perl compiles the
250entire program before executing it (except for C<BEGIN> blocks, which
251execute at compile time).
252
253=item *
254
255The arguments are available via @ARGV, not $1, $2, etc.
256
257=item *
258
259The environment is not automatically made available as separate scalar
260variables.
261
262=back
263
264=head2 Perl Traps
265
266Practicing Perl Programmers should take note of the following:
267
268=over 4
269
270=item *
271
272Remember that many operations behave differently in a list
273context than they do in a scalar one. See L<perldata> for details.
274
275=item *
276
68dc0745 277Avoid barewords if you can, especially all lowercase ones.
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278You can't tell by just looking at it whether a bareword is
279a function or a string. By using quotes on strings and
5f05dabc 280parentheses on function calls, you won't ever get them confused.
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281
282=item *
283
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284You cannot discern from mere inspection which builtins
285are unary operators (like chop() and chdir())
a0d0e21e 286and which are list operators (like print() and unlink()).
5f05dabc 287(User-defined subroutines can be B<only> list operators, never
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288unary ones.) See L<perlop>.
289
290=item *
291
748a9306 292People have a hard time remembering that some functions
a0d0e21e 293default to $_, or @ARGV, or whatever, but that others which
54310121 294you might expect to do not.
a0d0e21e 295
6dbacca0 296=item *
a0d0e21e 297
8b0a4b75 298The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
5f05dabc 299operation on that handle. The data read is assigned to $_ only if the
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300file read is the sole condition in a while loop:
301
302 while (<FH>) { }
54310121 303 while (defined($_ = <FH>)) { }..
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304 <FH>; # data discarded!
305
6dbacca0 306=item *
748a9306 307
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308Remember not to use "C<=>" when you need "C<=~>";
309these two constructs are quite different:
310
311 $x = /foo/;
312 $x =~ /foo/;
313
314=item *
315
54310121 316The C<do {}> construct isn't a real loop that you can use
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317loop control on.
318
319=item *
320
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321Use C<my()> for local variables whenever you can get away with
322it (but see L<perlform> for where you can't).
323Using C<local()> actually gives a local value to a global
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324variable, which leaves you open to unforeseen side-effects
325of dynamic scoping.
326
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327=item *
328
329If you localize an exported variable in a module, its exported value will
330not change. The local name becomes an alias to a new value but the
331external name is still an alias for the original.
332
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333=back
334
5f05dabc 335=head2 Perl4 to Perl5 Traps
a0d0e21e 336
54310121 337Practicing Perl4 Programmers should take note of the following
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338Perl4-to-Perl5 specific traps.
339
340They're crudely ordered according to the following list:
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341
342=over 4
343
6dbacca0 344=item Discontinuance, Deprecation, and BugFix traps
a0d0e21e 345
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346Anything that's been fixed as a perl4 bug, removed as a perl4 feature
347or deprecated as a perl4 feature with the intent to encourage usage of
348some other perl5 feature.
a0d0e21e 349
6dbacca0 350=item Parsing Traps
748a9306 351
6dbacca0 352Traps that appear to stem from the new parser.
a0d0e21e 353
6dbacca0 354=item Numerical Traps
a0d0e21e 355
6dbacca0 356Traps having to do with numerical or mathematical operators.
a0d0e21e 357
6dbacca0 358=item General data type traps
a0d0e21e 359
6dbacca0 360Traps involving perl standard data types.
a0d0e21e 361
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362=item Context Traps - scalar, list contexts
363
364Traps related to context within lists, scalar statements/declarations.
365
366=item Precedence Traps
367
368Traps related to the precedence of parsing, evaluation, and execution of
369code.
370
371=item General Regular Expression Traps using s///, etc.
372
373Traps related to the use of pattern matching.
374
375=item Subroutine, Signal, Sorting Traps
376
377Traps related to the use of signals and signal handlers, general subroutines,
378and sorting, along with sorting subroutines.
379
380=item OS Traps
381
382OS-specific traps.
383
384=item DBM Traps
385
386Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
387
388=item Unclassified Traps
389
390Everything else.
391
392=back
393
394If you find an example of a conversion trap that is not listed here,
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395please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
396Also note that at least some of these can be caught with B<-w>.
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397
398=head2 Discontinuance, Deprecation, and BugFix traps
399
400Anything that has been discontinued, deprecated, or fixed as
54310121 401a bug from perl4.
a0d0e21e 402
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403=over 4
404
54310121 405=item * Discontinuance
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406
407Symbols starting with "_" are no longer forced into package main, except
408for C<$_> itself (and C<@_>, etc.).
409
410 package test;
411 $_legacy = 1;
cb1a09d0 412
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413 package main;
414 print "\$_legacy is ",$_legacy,"\n";
54310121 415
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416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
418
54310121 419=item * Deprecation
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420
421Double-colon is now a valid package separator in a variable name. Thus these
5f05dabc 422behave differently in perl4 vs. perl5, because the packages don't exist.
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423
424 $a=1;$b=2;$c=3;$var=4;
425 print "$a::$b::$c ";
cb1a09d0 426 print "$var::abc::xyz\n";
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427
428 # perl4 prints: 1::2::3 4::abc::xyz
429 # perl5 prints: 3
cb1a09d0 430
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431Given that C<::> is now the preferred package delimiter, it is debatable
432whether this should be classed as a bug or not.
433(The older package delimiter, ' ,is used here)
cb1a09d0 434
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435 $x = 10 ;
436 print "x=${'x}\n" ;
54310121 437
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438 # perl4 prints: x=10
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
a0d0e21e 440
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441You can avoid this problem, and remain compatible with perl4, if you
442always explicitly include the package name:
443
444 $x = 10 ;
445 print "x=${main'x}\n" ;
446
54310121 447Also see precedence traps, for parsing C<$:>.
a0d0e21e 448
6dbacca0 449=item * BugFix
a0d0e21e 450
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451The second and third arguments of C<splice()> are now evaluated in scalar
452context (as the Camel says) rather than list context.
a0d0e21e 453
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454 sub sub1{return(0,2) } # return a 2-element list
455 sub sub2{ return(1,2,3)} # return a 3-element list
54310121 456 @a1 = ("a","b","c","d","e");
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457 @a2 = splice(@a1,&sub1,&sub2);
458 print join(' ',@a2),"\n";
54310121 459
6dbacca0 460 # perl4 prints: a b
54310121 461 # perl5 prints: c d e
a0d0e21e 462
54310121 463=item * Discontinuance
a0d0e21e 464
6dbacca0 465You can't do a C<goto> into a block that is optimized away. Darn.
a0d0e21e 466
6dbacca0 467 goto marker1;
a0d0e21e 468
54310121 469 for(1){
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470 marker1:
471 print "Here I is!\n";
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472 }
473
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474 # perl4 prints: Here I is!
475 # perl5 dumps core (SEGV)
476
54310121 477=item * Discontinuance
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478
479It is no longer syntactically legal to use whitespace as the name
480of a variable, or as a delimiter for any kind of quote construct.
54310121 481Double darn.
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482
483 $a = ("foo bar");
484 $b = q baz ;
485 print "a is $a, b is $b\n";
54310121 486
6dbacca0 487 # perl4 prints: a is foo bar, b is baz
54310121 488 # perl5 errors: Bareword found where operator expected
5e378fdf 489
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490=item * Discontinuance
491
492The archaic while/if BLOCK BLOCK syntax is no longer supported.
493
494 if { 1 } {
495 print "True!";
496 }
497 else {
498 print "False!";
499 }
54310121 500
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501 # perl4 prints: True!
502 # perl5 errors: syntax error at test.pl line 1, near "if {"
503
504=item * BugFix
505
506The C<**> operator now binds more tightly than unary minus.
507It was documented to work this way before, but didn't.
508
509 print -4**2,"\n";
54310121 510
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511 # perl4 prints: 16
512 # perl5 prints: -16
513
54310121 514=item * Discontinuance
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515
516The meaning of C<foreach{}> has changed slightly when it is iterating over a
517list which is not an array. This used to assign the list to a
518temporary array, but no longer does so (for efficiency). This means
519that you'll now be iterating over the actual values, not over copies of
520the values. Modifications to the loop variable can change the original
521values.
522
523 @list = ('ab','abc','bcd','def');
524 foreach $var (grep(/ab/,@list)){
525 $var = 1;
526 }
527 print (join(':',@list));
54310121 528
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529 # perl4 prints: ab:abc:bcd:def
530 # perl5 prints: 1:1:bcd:def
531
532To retain Perl4 semantics you need to assign your list
54310121 533explicitly to a temporary array and then iterate over that. For
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534example, you might need to change
535
536 foreach $var (grep(/ab/,@list)){
537
538to
539
540 foreach $var (@tmp = grep(/ab/,@list)){
541
542Otherwise changing $var will clobber the values of @list. (This most often
543happens when you use C<$_> for the loop variable, and call subroutines in
544the loop that don't properly localize C<$_>.)
545
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546=item * Discontinuance
547
548C<split> with no arguments now behaves like C<split ' '> (which doesn't
549return an initial null field if $_ starts with whitespace), it used to
550behave like C<split /\s+/> (which does).
551
552 $_ = ' hi mom';
553 print join(':', split);
554
555 # perl4 prints: :hi:mom
556 # perl5 prints: hi:mom
557
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558=item * BugFix
559
9607fc9c 560Perl 4 would ignore any text which was attached to an B<-e> switch,
55497cff 561always taking the code snippet from the following arg. Additionally, it
9607fc9c 562would silently accept an B<-e> switch without a following arg. Both of
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563these behaviors have been fixed.
564
565 perl -e'print "attached to -e"' 'print "separate arg"'
54310121 566
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567 # perl4 prints: separate arg
568 # perl5 prints: attached to -e
54310121 569
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570 perl -e
571
572 # perl4 prints:
573 # perl5 dies: No code specified for -e.
574
575=item * Discontinuance
576
577In Perl 4 the return value of C<push> was undocumented, but it was
578actually the last value being pushed onto the target list. In Perl 5
579the return value of C<push> is documented, but has changed, it is the
580number of elements in the resulting list.
581
582 @x = ('existing');
583 print push(@x, 'first new', 'second new');
54310121 584
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585 # perl4 prints: second new
586 # perl5 prints: 3
587
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588=item * Discontinuance
589
590In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in
591Perl code were silently allowed, although they could cause (mysterious!)
592failures in certain constructs, particularly here documents. Now,
593C<'\r'> characters cause an immediate fatal error. (Note: In this
594example, the notation B<\015> represents the incorrect line
595ending. Depending upon your text viewer, it will look different.)
596
597 print "foo";\015
598 print "bar";
599
600 # perl4 prints: foobar
601 # perl5.003 prints: foobar
602 # perl5.004 dies: Illegal character \015 (carriage return)
603
604See L<perldiag> for full details.
605
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606=item * Deprecation
607
608Some error messages will be different.
609
54310121 610=item * Discontinuance
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611
612Some bugs may have been inadvertently removed. :-)
613
614=back
615
616=head2 Parsing Traps
617
618Perl4-to-Perl5 traps from having to do with parsing.
619
620=over 4
621
622=item * Parsing
623
624Note the space between . and =
625
626 $string . = "more string";
627 print $string;
54310121 628
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629 # perl4 prints: more string
630 # perl5 prints: syntax error at - line 1, near ". ="
631
632=item * Parsing
633
634Better parsing in perl 5
635
636 sub foo {}
637 &foo
638 print("hello, world\n");
54310121 639
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640 # perl4 prints: hello, world
641 # perl5 prints: syntax error
642
643=item * Parsing
644
645"if it looks like a function, it is a function" rule.
646
647 print
648 ($foo == 1) ? "is one\n" : "is zero\n";
54310121 649
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650 # perl4 prints: is zero
651 # perl5 warns: "Useless use of a constant in void context" if using -w
652
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653=item * Parsing
654
655String interpolation of the C<$#array> construct differs when braces
656are to used around the name.
657
658 @ = (1..3);
659 print "${#a}";
660
661 # perl4 prints: 2
662 # perl5 fails with syntax error
663
664 @ = (1..3);
665 print "$#{a}";
666
667 # perl4 prints: {a}
668 # perl5 prints: 2
669
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670=back
671
672=head2 Numerical Traps
673
674Perl4-to-Perl5 traps having to do with numerical operators,
675operands, or output from same.
676
677=over 5
678
679=item * Numerical
680
681Formatted output and significant digits
682
683 print 7.373504 - 0, "\n";
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684 printf "%20.18f\n", 7.373504 - 0;
685
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686 # Perl4 prints:
687 7.375039999999996141
688 7.37503999999999614
54310121 689
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690 # Perl5 prints:
691 7.373504
692 7.37503999999999614
693
694=item * Numerical
695
5f05dabc 696This specific item has been deleted. It demonstrated how the auto-increment
5e378fdf 697operator would not catch when a number went over the signed int limit. Fixed
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698in version 5.003_04. But always be wary when using large integers.
699If in doubt:
6dbacca0 700
5e378fdf 701 use Math::BigInt;
6dbacca0 702
54310121 703=item * Numerical
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704
705Assignment of return values from numeric equality tests
706does not work in perl5 when the test evaluates to false (0).
707Logical tests now return an null, instead of 0
a6006777 708
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709 $p = ($test == 1);
710 print $p,"\n";
a6006777 711
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712 # perl4 prints: 0
713 # perl5 prints:
714
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715Also see L<"General Regular Expression Traps using s///, etc.">
716for another example of this new feature...
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717
718=back
719
720=head2 General data type traps
721
722Perl4-to-Perl5 traps involving most data-types, and their usage
723within certain expressions and/or context.
724
725=over 5
726
727=item * (Arrays)
728
729Negative array subscripts now count from the end of the array.
730
731 @a = (1, 2, 3, 4, 5);
732 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
54310121 733
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PP
734 # perl4 prints: The third element of the array is 4 also expressed as
735 # perl5 prints: The third element of the array is 4 also expressed as 4
736
737=item * (Arrays)
738
739Setting C<$#array> lower now discards array elements, and makes them
740impossible to recover.
741
54310121 742 @a = (a,b,c,d,e);
6dbacca0 743 print "Before: ",join('',@a);
54310121 744 $#a =1;
6dbacca0
PP
745 print ", After: ",join('',@a);
746 $#a =3;
747 print ", Recovered: ",join('',@a),"\n";
54310121 748
6dbacca0
PP
749 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
750 # perl5 prints: Before: abcde, After: ab, Recovered: ab
751
752=item * (Hashes)
753
754Hashes get defined before use
755
54310121 756 local($s,@a,%h);
6dbacca0
PP
757 die "scalar \$s defined" if defined($s);
758 die "array \@a defined" if defined(@a);
759 die "hash \%h defined" if defined(%h);
54310121 760
6dbacca0
PP
761 # perl4 prints:
762 # perl5 dies: hash %h defined
763
764=item * (Globs)
765
766glob assignment from variable to variable will fail if the assigned
767variable is localized subsequent to the assignment
768
769 @a = ("This is Perl 4");
770 *b = *a;
771 local(@a);
772 print @b,"\n";
54310121 773
6dbacca0
PP
774 # perl4 prints: This is Perl 4
775 # perl5 prints:
54310121 776
a3cb178b 777=item * (Globs)
54310121 778
a3cb178b
GS
779Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
780it undefines the associated scalar (but may have other side effects
781including SEGVs).
5e378fdf 782
6dbacca0
PP
783=item * (Scalar String)
784
785Changes in unary negation (of strings)
786This change effects both the return value and what it
787does to auto(magic)increment.
788
789 $x = "aaa";
790 print ++$x," : ";
791 print -$x," : ";
792 print ++$x,"\n";
54310121 793
6dbacca0
PP
794 # perl4 prints: aab : -0 : 1
795 # perl5 prints: aab : -aab : aac
796
797=item * (Constants)
798
799perl 4 lets you modify constants:
800
801 $foo = "x";
802 &mod($foo);
803 for ($x = 0; $x < 3; $x++) {
804 &mod("a");
805 }
806 sub mod {
807 print "before: $_[0]";
808 $_[0] = "m";
809 print " after: $_[0]\n";
810 }
54310121 811
6dbacca0
PP
812 # perl4:
813 # before: x after: m
814 # before: a after: m
815 # before: m after: m
816 # before: m after: m
54310121 817
6dbacca0
PP
818 # Perl5:
819 # before: x after: m
820 # Modification of a read-only value attempted at foo.pl line 12.
821 # before: a
822
823=item * (Scalars)
824
825The behavior is slightly different for:
826
827 print "$x", defined $x
54310121 828
6dbacca0
PP
829 # perl 4: 1
830 # perl 5: <no output, $x is not called into existence>
831
832=item * (Variable Suicide)
833
834Variable suicide behavior is more consistent under Perl 5.
aa689395 835Perl5 exhibits the same behavior for hashes and scalars,
5f05dabc 836that perl4 exhibits for only scalars.
6dbacca0
PP
837
838 $aGlobal{ "aKey" } = "global value";
839 print "MAIN:", $aGlobal{"aKey"}, "\n";
840 $GlobalLevel = 0;
841 &test( *aGlobal );
842
843 sub test {
844 local( *theArgument ) = @_;
845 local( %aNewLocal ); # perl 4 != 5.001l,m
54310121 846 $aNewLocal{"aKey"} = "this should never appear";
6dbacca0
PP
847 print "SUB: ", $theArgument{"aKey"}, "\n";
848 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
849 $GlobalLevel++;
850 if( $GlobalLevel<4 ) {
851 &test( *aNewLocal );
852 }
853 }
54310121 854
6dbacca0
PP
855 # Perl4:
856 # MAIN:global value
857 # SUB: global value
858 # SUB: level 0
859 # SUB: level 1
860 # SUB: level 2
54310121 861
6dbacca0
PP
862 # Perl5:
863 # MAIN:global value
864 # SUB: global value
865 # SUB: this should never appear
866 # SUB: this should never appear
867 # SUB: this should never appear
868
84dc3c4d 869=back
6dbacca0
PP
870
871=head2 Context Traps - scalar, list contexts
872
873=over 5
874
875=item * (list context)
876
877The elements of argument lists for formats are now evaluated in list
878context. This means you can interpolate list values now.
879
880 @fmt = ("foo","bar","baz");
881 format STDOUT=
882 @<<<<< @||||| @>>>>>
883 @fmt;
884 .
54310121
PP
885 write;
886
6dbacca0
PP
887 # perl4 errors: Please use commas to separate fields in file
888 # perl5 prints: foo bar baz
889
890=item * (scalar context)
891
54310121
PP
892The C<caller()> function now returns a false value in a scalar context
893if there is no caller. This lets library files determine if they're
6dbacca0
PP
894being required.
895
896 caller() ? (print "You rang?\n") : (print "Got a 0\n");
54310121 897
6dbacca0
PP
898 # perl4 errors: There is no caller
899 # perl5 prints: Got a 0
5e378fdf 900
6dbacca0
PP
901=item * (scalar context)
902
903The comma operator in a scalar context is now guaranteed to give a
904scalar context to its arguments.
905
906 @y= ('a','b','c');
907 $x = (1, 2, @y);
908 print "x = $x\n";
54310121 909
6dbacca0
PP
910 # Perl4 prints: x = c # Thinks list context interpolates list
911 # Perl5 prints: x = 3 # Knows scalar uses length of list
912
913=item * (list, builtin)
914
915C<sprintf()> funkiness (array argument converted to scalar array count)
916This test could be added to t/op/sprintf.t
917
918 @z = ('%s%s', 'foo', 'bar');
919 $x = sprintf(@z);
920 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
54310121 921
6dbacca0
PP
922 # perl4 prints: ok 2
923 # perl5 prints: not ok 2
924
925C<printf()> works fine, though:
926
927 printf STDOUT (@z);
54310121
PP
928 print "\n";
929
6dbacca0
PP
930 # perl4 prints: foobar
931 # perl5 prints: foobar
932
933Probably a bug.
934
935=back
936
937=head2 Precedence Traps
938
939Perl4-to-Perl5 traps involving precedence order.
940
f4b17341
GS
941Perl 4 has almost the same precedence rules as Perl 5 for the operators
942that they both have. Perl 4 however, seems to have had some
943inconsistencies that made the behavior differ from what was documented.
944
84dc3c4d
PP
945=over 5
946
5e378fdf
PP
947=item * Precedence
948
8dbef698
JM
949LHS vs. RHS of any assignment operator. LHS is evaluated first
950in perl4, second in perl5; this can affect the relationship
951between side-effects in sub-expressions.
5e378fdf
PP
952
953 @arr = ( 'left', 'right' );
954 $a{shift @arr} = shift @arr;
955 print join( ' ', keys %a );
956
957 # perl4 prints: left
958 # perl5 prints: right
959
960=item * Precedence
6dbacca0
PP
961
962These are now semantic errors because of precedence:
963
964 @list = (1,2,3,4,5);
965 %map = ("a",1,"b",2,"c",3,"d",4);
966 $n = shift @list + 2; # first item in list plus 2
967 print "n is $n, ";
968 $m = keys %map + 2; # number of items in hash plus 2
969 print "m is $m\n";
54310121 970
6dbacca0
PP
971 # perl4 prints: n is 3, m is 6
972 # perl5 errors and fails to compile
973
974=item * Precedence
a0d0e21e 975
4633a7c4
LW
976The precedence of assignment operators is now the same as the precedence
977of assignment. Perl 4 mistakenly gave them the precedence of the associated
978operator. So you now must parenthesize them in expressions like
979
980 /foo/ ? ($a += 2) : ($a -= 2);
a6006777 981
4633a7c4
LW
982Otherwise
983
6dbacca0 984 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
985
986would be erroneously parsed as
987
988 (/foo/ ? $a += 2 : $a) -= 2;
989
990On the other hand,
991
54310121 992 $a += /foo/ ? 1 : 2;
4633a7c4
LW
993
994now works as a C programmer would expect.
995
6dbacca0 996=item * Precedence
4633a7c4 997
6dbacca0 998 open FOO || die;
a0d0e21e 999
5f05dabc
PP
1000is now incorrect. You need parentheses around the filehandle.
1001Otherwise, perl5 leaves the statement as its default precedence:
a0d0e21e 1002
6dbacca0 1003 open(FOO || die);
54310121 1004
6dbacca0
PP
1005 # perl4 opens or dies
1006 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 1007
6dbacca0 1008=item * Precedence
a0d0e21e 1009
6dbacca0
PP
1010perl4 gives the special variable, C<$:> precedence, where perl5
1011treats C<$::> as main C<package>
a0d0e21e 1012
6dbacca0 1013 $a = "x"; print "$::a";
54310121 1014
6dbacca0
PP
1015 # perl 4 prints: -:a
1016 # perl 5 prints: x
5e378fdf 1017
6dbacca0 1018=item * Precedence
a0d0e21e 1019
f4b17341
GS
1020perl4 had buggy precedence for the file test operators vis-a-vis
1021the assignment operators. Thus, although the precedence table
1022for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1023C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1024In perl5, the precedence is as documented.
54310121
PP
1025
1026 -e $foo .= "q"
a0d0e21e 1027
6dbacca0
PP
1028 # perl4 prints: no output
1029 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 1030
f4b17341
GS
1031=item * Precedence
1032
1033In perl4, keys(), each() and values() were special high-precedence operators
1034that operated on a single hash, but in perl5, they are regular named unary
1035operators. As documented, named unary operators have lower precedence
1036than the arithmetic and concatenation operators C<+ - .>, but the perl4
1037variants of these operators actually bind tighter than C<+ - .>.
1038Thus, for:
1039
1040 %foo = 1..10;
1041 print keys %foo - 1
1042
1043 # perl4 prints: 4
1044 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1045
1046The perl4 behavior was probably more useful, if less consistent.
1047
6dbacca0
PP
1048=back
1049
1050=head2 General Regular Expression Traps using s///, etc.
1051
1052All types of RE traps.
1053
1054=over 5
1055
1056=item * Regular Expression
1057
1058C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
54310121 1059interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
6dbacca0
PP
1060'$' in string)
1061
1062 $a=1;$b=2;
1063 $string = '1 2 $a $b';
1064 $string =~ s'$a'$b';
1065 print $string,"\n";
54310121 1066
6dbacca0
PP
1067 # perl4 prints: $b 2 $a $b
1068 # perl5 prints: 1 2 $a $b
1069
1070=item * Regular Expression
a0d0e21e
LW
1071
1072C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
1073regular expression. (Once the scope of a block is left for the sub, the
1074state of the searched string is lost)
1075
1076 $_ = "ababab";
1077 while(m/ab/g){
1078 &doit("blah");
1079 }
1080 sub doit{local($_) = shift; print "Got $_ "}
54310121 1081
6dbacca0
PP
1082 # perl4 prints: blah blah blah
1083 # perl5 prints: infinite loop blah...
1084
1085=item * Regular Expression
1086
68dc0745
PP
1087Currently, if you use the C<m//o> qualifier on a regular expression
1088within an anonymous sub, I<all> closures generated from that anonymous
1089sub will use the regular expression as it was compiled when it was used
1090the very first time in any such closure. For instance, if you say
1091
1092 sub build_match {
1093 my($left,$right) = @_;
1094 return sub { $_[0] =~ /$left stuff $right/o; };
1095 }
1096
1097build_match() will always return a sub which matches the contents of
1098C<$left> and C<$right> as they were the I<first> time that build_match()
1099was called, not as they are in the current call.
1100
1101This is probably a bug, and may change in future versions of Perl.
1102
1103=item * Regular Expression
1104
6dbacca0
PP
1105If no parentheses are used in a match, Perl4 sets C<$+> to
1106the whole match, just like C<$&>. Perl5 does not.
1107
1108 "abcdef" =~ /b.*e/;
1109 print "\$+ = $+\n";
54310121 1110
6dbacca0
PP
1111 # perl4 prints: bcde
1112 # perl5 prints:
1113
1114=item * Regular Expression
1115
1116substitution now returns the null string if it fails
1117
1118 $string = "test";
1119 $value = ($string =~ s/foo//);
1120 print $value, "\n";
54310121 1121
6dbacca0
PP
1122 # perl4 prints: 0
1123 # perl5 prints:
1124
1125Also see L<Numerical Traps> for another example of this new feature.
1126
1127=item * Regular Expression
1128
54310121
PP
1129C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1130backtick expansion
6dbacca0
PP
1131
1132 $string = "";
1133 $string =~ s`^`hostname`;
1134 print $string, "\n";
54310121 1135
6dbacca0
PP
1136 # perl4 prints: <the local hostname>
1137 # perl5 prints: hostname
1138
1139=item * Regular Expression
1140
1141Stricter parsing of variables used in regular expressions
1142
1143 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
54310121 1144
6dbacca0
PP
1145 # perl4: compiles w/o error
1146 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1147
1148an added component of this example, apparently from the same script, is
1149the actual value of the s'd string after the substitution.
1150C<[$opt]> is a character class in perl4 and an array subscript in perl5
1151
54310121 1152 $grpc = 'a';
6dbacca0
PP
1153 $opt = 'r';
1154 $_ = 'bar';
1155 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1156 print ;
54310121 1157
6dbacca0
PP
1158 # perl4 prints: foo
1159 # perl5 prints: foobar
1160
1161=item * Regular Expression
1162
1163Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1164repeatedly, like C</x/> or C<m!x!>.
1165
1166 $test = "once";
1167 sub match { $test =~ m?once?; }
1168 &match();
1169 if( &match() ) {
1170 # m?x? matches more then once
1171 print "perl4\n";
54310121 1172 } else {
6dbacca0 1173 # m?x? matches only once
54310121 1174 print "perl5\n";
6dbacca0 1175 }
54310121 1176
6dbacca0
PP
1177 # perl4 prints: perl4
1178 # perl5 prints: perl5
a0d0e21e 1179
a0d0e21e 1180
6dbacca0
PP
1181=back
1182
1183=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1184
6dbacca0
PP
1185The general group of Perl4-to-Perl5 traps having to do with
1186Signals, Sorting, and their related subroutines, as well as
1187general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1188
6dbacca0 1189=over 5
a0d0e21e 1190
6dbacca0 1191=item * (Signals)
a0d0e21e 1192
6dbacca0
PP
1193Barewords that used to look like strings to Perl will now look like subroutine
1194calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1195
6dbacca0
PP
1196 sub SeeYa { warn"Hasta la vista, baby!" }
1197 $SIG{'TERM'} = SeeYa;
1198 print "SIGTERM is now $SIG{'TERM'}\n";
54310121 1199
6dbacca0
PP
1200 # perl4 prints: SIGTERM is main'SeeYa
1201 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1202
6dbacca0 1203Use B<-w> to catch this one
a0d0e21e 1204
6dbacca0 1205=item * (Sort Subroutine)
a0d0e21e 1206
6dbacca0 1207reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1208
6dbacca0 1209 sub reverse{ print "yup "; $a <=> $b }
54310121
PP
1210 print sort reverse a,b,c;
1211
6dbacca0 1212 # perl4 prints: yup yup yup yup abc
54310121 1213 # perl5 prints: abc
a0d0e21e 1214
b996531f
PP
1215=item * warn() won't let you specify a filehandle.
1216
1217Although it _always_ printed to STDERR, warn() would let you specify a
1218filehandle in perl4. With perl5 it does not.
5e378fdf
PP
1219
1220 warn STDERR "Foo!";
1221
1222 # perl4 prints: Foo!
54310121 1223 # perl5 prints: String found where operator expected
5e378fdf 1224
6dbacca0 1225=back
a0d0e21e 1226
6dbacca0
PP
1227=head2 OS Traps
1228
1229=over 5
1230
1231=item * (SysV)
1232
54310121
PP
1233Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1234within the signal handler function, each time a signal was handled with
1235perl4. With perl5, the reset is now done correctly. Any code relying
6dbacca0
PP
1236on the handler _not_ being reset will have to be reworked.
1237
a6006777 1238Since version 5.002, Perl uses sigaction() under SysV.
6dbacca0
PP
1239
1240 sub gotit {
54310121
PP
1241 print "Got @_... ";
1242 }
6dbacca0 1243 $SIG{'INT'} = 'gotit';
54310121 1244
6dbacca0
PP
1245 $| = 1;
1246 $pid = fork;
1247 if ($pid) {
1248 kill('INT', $pid);
1249 sleep(1);
1250 kill('INT', $pid);
54310121 1251 } else {
6dbacca0 1252 while (1) {sleep(10);}
54310121
PP
1253 }
1254
6dbacca0
PP
1255 # perl4 (HPUX) prints: Got INT...
1256 # perl5 (HPUX) prints: Got INT... Got INT...
1257
1258=item * (SysV)
1259
54310121
PP
1260Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1261the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
6dbacca0
PP
1262for append, it is impossible to overwrite information already in
1263the file.
1264
1265 open(TEST,">>seek.test");
54310121 1266 $start = tell TEST ;
6dbacca0
PP
1267 foreach(1 .. 9){
1268 print TEST "$_ ";
1269 }
1270 $end = tell TEST ;
1271 seek(TEST,$start,0);
1272 print TEST "18 characters here";
54310121 1273
6dbacca0
PP
1274 # perl4 (solaris) seek.test has: 18 characters here
1275 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1276
a0d0e21e 1277
a0d0e21e 1278
6dbacca0 1279=back
a0d0e21e 1280
6dbacca0 1281=head2 Interpolation Traps
a0d0e21e 1282
8b0a4b75
PP
1283Perl4-to-Perl5 traps having to do with how things get interpolated
1284within certain expressions, statements, contexts, or whatever.
1285
6dbacca0 1286=over 5
a0d0e21e 1287
6dbacca0 1288=item * Interpolation
a0d0e21e 1289
6dbacca0
PP
1290@ now always interpolates an array in double-quotish strings.
1291
54310121
PP
1292 print "To: someone@somewhere.com\n";
1293
6dbacca0 1294 # perl4 prints: To:someone@somewhere.com
9607fc9c 1295 # perl5 errors : In string, @somewhere now must be written as \@somewhere
6dbacca0
PP
1296
1297=item * Interpolation
1298
6dbacca0
PP
1299Double-quoted strings may no longer end with an unescaped $ or @.
1300
1301 $foo = "foo$";
1302 $bar = "bar@";
1303 print "foo is $foo, bar is $bar\n";
54310121 1304
6dbacca0
PP
1305 # perl4 prints: foo is foo$, bar is bar@
1306 # perl5 errors: Final $ should be \$ or $name
1307
1308Note: perl5 DOES NOT error on the terminating @ in $bar
1309
1310=item * Interpolation
a0d0e21e 1311
8b0a4b75
PP
1312Perl now sometimes evaluates arbitrary expressions inside braces that occur
1313within double quotes (usually when the opening brace is preceded by C<$>
1314or C<@>).
1315
1316 @www = "buz";
1317 $foo = "foo";
1318 $bar = "bar";
1319 sub foo { return "bar" };
1320 print "|@{w.w.w}|${main'foo}|";
1321
1322 # perl4 prints: |@{w.w.w}|foo|
1323 # perl5 prints: |buz|bar|
1324
1325Note that you can C<use strict;> to ward off such trappiness under perl5.
1326
1327=item * Interpolation
1328
748a9306 1329The construct "this is $$x" used to interpolate the pid at that
6dbacca0 1330point, but now apparently tries to dereference C<$x>. C<$$> by itself still
748a9306
LW
1331works fine, however.
1332
6dbacca0 1333 print "this is $$x\n";
748a9306 1334
6dbacca0
PP
1335 # perl4 prints: this is XXXx (XXX is the current pid)
1336 # perl5 prints: this is
1337
1338=item * Interpolation
1339
54310121
PP
1340Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1341C<$>'s to be protected in the specification of the hash name, or both curlies
6dbacca0
PP
1342to be protected. If both curlies are protected, the result will be compatible
1343with perl4 and perl5. This is a very common practice, and should be changed
1344to use the block form of C<eval{}> if possible.
c07a80fd 1345
6dbacca0
PP
1346 $hashname = "foobar";
1347 $key = "baz";
1348 $value = 1234;
1349 eval "\$$hashname{'$key'} = q|$value|";
1350 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1351
1352 # perl4 prints: Yup
1353 # perl5 prints: Nope
1354
1355Changing
1356
1357 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1358
1359to
1360
6dbacca0 1361 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1362
6dbacca0 1363causes the following result:
c07a80fd 1364
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1365 # perl4 prints: Nope
1366 # perl5 prints: Yup
c07a80fd 1367
6dbacca0 1368or, changing to
a0d0e21e 1369
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PP
1370 eval "\$$hashname\{'$key'\} = q|$value|";
1371
1372causes the following result:
1373
1374 # perl4 prints: Yup
1375 # perl5 prints: Yup
1376 # and is compatible for both versions
1377
1378
1379=item * Interpolation
1380
1381perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1382
1383 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
54310121 1384
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PP
1385 # perl4 prints: This is not perl5
1386 # perl5 prints: This is perl5
1387
1388=item * Interpolation
1389
54310121 1390You also have to be careful about array references.
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PP
1391
1392 print "$foo{"
1393
1394 perl 4 prints: {
1395 perl 5 prints: syntax error
1396
1397=item * Interpolation
1398
1399Similarly, watch out for:
1400
1401 $foo = "array";
1402 print "\$$foo{bar}\n";
54310121 1403
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1404 # perl4 prints: $array{bar}
1405 # perl5 prints: $
1406
1407Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1408happy just to expand $foo to "array" by itself. Watch out for this
1409especially in C<eval>'s.
1410
1411=item * Interpolation
1412
1413C<qq()> string passed to C<eval>
1414
1415 eval qq(
1416 foreach \$y (keys %\$x\) {
1417 \$count++;
1418 }
1419 );
54310121 1420
6dbacca0 1421 # perl4 runs this ok
54310121 1422 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1423
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PP
1424=back
1425
1426=head2 DBM Traps
1427
1428General DBM traps.
1429
1430=over 5
1431
1432=item * DBM
1433
1434Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1435may cause the same script, run under perl5, to fail. The build of perl5
1436must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1437to function properly without C<tie>'ing to an extension dbm implementation.
1438
1439 dbmopen (%dbm, "file", undef);
1440 print "ok\n";
1441
1442 # perl4 prints: ok
1443 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1444
1445
1446=item * DBM
1447
1448Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1449may cause the same script, run under perl5, to fail. The error generated
1450when exceeding the limit on the key/value size will cause perl5 to exit
1451immediately.
1452
1453 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1454 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1455 print "YUP\n";
1456
1457 # perl4 prints:
1458 dbm store returned -1, errno 28, key "trap" at - line 3.
1459 YUP
1460
1461 # perl5 prints:
1462 dbm store returned -1, errno 28, key "trap" at - line 3.
a0d0e21e
LW
1463
1464=back
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1465
1466=head2 Unclassified Traps
1467
1468Everything else.
1469
84dc3c4d
PP
1470=over 5
1471
5db417f7 1472=item * C<require>/C<do> trap using returned value
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1473
1474If the file doit.pl has:
1475
1476 sub foo {
1477 $rc = do "./do.pl";
1478 return 8;
54310121 1479 }
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1480 print &foo, "\n";
1481
1482And the do.pl file has the following single line:
1483
1484 return 3;
1485
1486Running doit.pl gives the following:
1487
1488 # perl 4 prints: 3 (aborts the subroutine early)
54310121 1489 # perl 5 prints: 8
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PP
1490
1491Same behavior if you replace C<do> with C<require>.
1492
5db417f7
TB
1493=item * C<split> on empty string with LIMIT specified
1494
1495 $string = '';
1496 @list = split(/foo/, $string, 2)
1497
1498Perl4 returns a one element list containing the empty string but Perl5
1499returns an empty list.
1500
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PP
1501=back
1502
54310121 1503As always, if any of these are ever officially declared as bugs,
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1504they'll be fixed and removed.
1505