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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
54310121 24allows you to refer to special variables (like C<$/>) with names (like
19799a22 25$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 $RS to a pattern, only a string.
137
138=item *
139
140When in doubt, run the B<awk> construct through B<a2p> and see what it
141gives you.
142
143=back
144
145=head2 C Traps
146
147Cerebral C programmers should take note of the following:
148
149=over 4
150
151=item *
152
153Curly brackets are required on C<if>'s and C<while>'s.
154
155=item *
156
157You must use C<elsif> rather than C<else if>.
158
159=item *
160
54310121 161The C<break> and C<continue> keywords from C become in
a0d0e21e 162Perl C<last> and C<next>, respectively.
19799a22 163Unlike in C, these do I<not> work within a C<do { } while> construct.
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164
165=item *
166
167There's no switch statement. (But it's easy to build one on the fly.)
168
169=item *
170
5db417f7 171Variables begin with "$", "@" 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
19799a22 308Remember not to use C<=> when you need C<=~>;
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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 $a[-2] \n";
54310121 733
6dbacca0
PP
734 # perl4 prints: The third element of the array is 4 also expressed as
735 # perl5 prints: The third element of the array is 4 also expressed as 4
736
737=item * (Arrays)
738
739Setting C<$#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
475342a6
GS
764Perl will now generate a warning when it sees defined(@a) and
765defined(%h).
766
6dbacca0
PP
767=item * (Globs)
768
769glob assignment from variable to variable will fail if the assigned
770variable is localized subsequent to the assignment
771
772 @a = ("This is Perl 4");
773 *b = *a;
774 local(@a);
775 print @b,"\n";
54310121 776
6dbacca0
PP
777 # perl4 prints: This is Perl 4
778 # perl5 prints:
54310121 779
a3cb178b 780=item * (Globs)
54310121 781
a3cb178b
GS
782Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
783it undefines the associated scalar (but may have other side effects
784including SEGVs).
5e378fdf 785
6dbacca0
PP
786=item * (Scalar String)
787
788Changes in unary negation (of strings)
789This change effects both the return value and what it
790does to auto(magic)increment.
791
792 $x = "aaa";
793 print ++$x," : ";
794 print -$x," : ";
795 print ++$x,"\n";
54310121 796
6dbacca0
PP
797 # perl4 prints: aab : -0 : 1
798 # perl5 prints: aab : -aab : aac
799
800=item * (Constants)
801
802perl 4 lets you modify constants:
803
804 $foo = "x";
805 &mod($foo);
806 for ($x = 0; $x < 3; $x++) {
807 &mod("a");
808 }
809 sub mod {
810 print "before: $_[0]";
811 $_[0] = "m";
812 print " after: $_[0]\n";
813 }
54310121 814
6dbacca0
PP
815 # perl4:
816 # before: x after: m
817 # before: a after: m
818 # before: m after: m
819 # before: m after: m
54310121 820
6dbacca0
PP
821 # Perl5:
822 # before: x after: m
823 # Modification of a read-only value attempted at foo.pl line 12.
824 # before: a
825
826=item * (Scalars)
827
828The behavior is slightly different for:
829
830 print "$x", defined $x
54310121 831
6dbacca0
PP
832 # perl 4: 1
833 # perl 5: <no output, $x is not called into existence>
834
835=item * (Variable Suicide)
836
837Variable suicide behavior is more consistent under Perl 5.
aa689395 838Perl5 exhibits the same behavior for hashes and scalars,
5f05dabc 839that perl4 exhibits for only scalars.
6dbacca0
PP
840
841 $aGlobal{ "aKey" } = "global value";
842 print "MAIN:", $aGlobal{"aKey"}, "\n";
843 $GlobalLevel = 0;
844 &test( *aGlobal );
845
846 sub test {
847 local( *theArgument ) = @_;
848 local( %aNewLocal ); # perl 4 != 5.001l,m
54310121 849 $aNewLocal{"aKey"} = "this should never appear";
6dbacca0
PP
850 print "SUB: ", $theArgument{"aKey"}, "\n";
851 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
852 $GlobalLevel++;
853 if( $GlobalLevel<4 ) {
854 &test( *aNewLocal );
855 }
856 }
54310121 857
6dbacca0
PP
858 # Perl4:
859 # MAIN:global value
860 # SUB: global value
861 # SUB: level 0
862 # SUB: level 1
863 # SUB: level 2
54310121 864
6dbacca0
PP
865 # Perl5:
866 # MAIN:global value
867 # SUB: global value
868 # SUB: this should never appear
869 # SUB: this should never appear
870 # SUB: this should never appear
871
84dc3c4d 872=back
6dbacca0
PP
873
874=head2 Context Traps - scalar, list contexts
875
876=over 5
877
878=item * (list context)
879
880The elements of argument lists for formats are now evaluated in list
881context. This means you can interpolate list values now.
882
883 @fmt = ("foo","bar","baz");
884 format STDOUT=
885 @<<<<< @||||| @>>>>>
886 @fmt;
887 .
54310121
PP
888 write;
889
6dbacca0
PP
890 # perl4 errors: Please use commas to separate fields in file
891 # perl5 prints: foo bar baz
892
893=item * (scalar context)
894
54310121
PP
895The C<caller()> function now returns a false value in a scalar context
896if there is no caller. This lets library files determine if they're
6dbacca0
PP
897being required.
898
899 caller() ? (print "You rang?\n") : (print "Got a 0\n");
54310121 900
6dbacca0
PP
901 # perl4 errors: There is no caller
902 # perl5 prints: Got a 0
5e378fdf 903
6dbacca0
PP
904=item * (scalar context)
905
906The comma operator in a scalar context is now guaranteed to give a
907scalar context to its arguments.
908
909 @y= ('a','b','c');
910 $x = (1, 2, @y);
911 print "x = $x\n";
54310121 912
6dbacca0
PP
913 # Perl4 prints: x = c # Thinks list context interpolates list
914 # Perl5 prints: x = 3 # Knows scalar uses length of list
915
916=item * (list, builtin)
917
918C<sprintf()> funkiness (array argument converted to scalar array count)
919This test could be added to t/op/sprintf.t
920
921 @z = ('%s%s', 'foo', 'bar');
922 $x = sprintf(@z);
923 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
54310121 924
6dbacca0
PP
925 # perl4 prints: ok 2
926 # perl5 prints: not ok 2
927
928C<printf()> works fine, though:
929
930 printf STDOUT (@z);
54310121
PP
931 print "\n";
932
6dbacca0
PP
933 # perl4 prints: foobar
934 # perl5 prints: foobar
935
936Probably a bug.
937
938=back
939
940=head2 Precedence Traps
941
942Perl4-to-Perl5 traps involving precedence order.
943
f4b17341
GS
944Perl 4 has almost the same precedence rules as Perl 5 for the operators
945that they both have. Perl 4 however, seems to have had some
946inconsistencies that made the behavior differ from what was documented.
947
84dc3c4d
PP
948=over 5
949
5e378fdf
PP
950=item * Precedence
951
8dbef698
JM
952LHS vs. RHS of any assignment operator. LHS is evaluated first
953in perl4, second in perl5; this can affect the relationship
954between side-effects in sub-expressions.
5e378fdf
PP
955
956 @arr = ( 'left', 'right' );
957 $a{shift @arr} = shift @arr;
958 print join( ' ', keys %a );
959
960 # perl4 prints: left
961 # perl5 prints: right
962
963=item * Precedence
6dbacca0
PP
964
965These are now semantic errors because of precedence:
966
967 @list = (1,2,3,4,5);
968 %map = ("a",1,"b",2,"c",3,"d",4);
969 $n = shift @list + 2; # first item in list plus 2
970 print "n is $n, ";
971 $m = keys %map + 2; # number of items in hash plus 2
972 print "m is $m\n";
54310121 973
6dbacca0
PP
974 # perl4 prints: n is 3, m is 6
975 # perl5 errors and fails to compile
976
977=item * Precedence
a0d0e21e 978
4633a7c4
LW
979The precedence of assignment operators is now the same as the precedence
980of assignment. Perl 4 mistakenly gave them the precedence of the associated
981operator. So you now must parenthesize them in expressions like
982
983 /foo/ ? ($a += 2) : ($a -= 2);
a6006777 984
4633a7c4
LW
985Otherwise
986
6dbacca0 987 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
988
989would be erroneously parsed as
990
991 (/foo/ ? $a += 2 : $a) -= 2;
992
993On the other hand,
994
54310121 995 $a += /foo/ ? 1 : 2;
4633a7c4
LW
996
997now works as a C programmer would expect.
998
6dbacca0 999=item * Precedence
4633a7c4 1000
6dbacca0 1001 open FOO || die;
a0d0e21e 1002
5f05dabc
PP
1003is now incorrect. You need parentheses around the filehandle.
1004Otherwise, perl5 leaves the statement as its default precedence:
a0d0e21e 1005
6dbacca0 1006 open(FOO || die);
54310121 1007
6dbacca0
PP
1008 # perl4 opens or dies
1009 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 1010
6dbacca0 1011=item * Precedence
a0d0e21e 1012
6dbacca0
PP
1013perl4 gives the special variable, C<$:> precedence, where perl5
1014treats C<$::> as main C<package>
a0d0e21e 1015
6dbacca0 1016 $a = "x"; print "$::a";
54310121 1017
6dbacca0
PP
1018 # perl 4 prints: -:a
1019 # perl 5 prints: x
5e378fdf 1020
6dbacca0 1021=item * Precedence
a0d0e21e 1022
f4b17341
GS
1023perl4 had buggy precedence for the file test operators vis-a-vis
1024the assignment operators. Thus, although the precedence table
1025for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1026C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1027In perl5, the precedence is as documented.
54310121
PP
1028
1029 -e $foo .= "q"
a0d0e21e 1030
6dbacca0
PP
1031 # perl4 prints: no output
1032 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 1033
f4b17341
GS
1034=item * Precedence
1035
1036In perl4, keys(), each() and values() were special high-precedence operators
1037that operated on a single hash, but in perl5, they are regular named unary
1038operators. As documented, named unary operators have lower precedence
1039than the arithmetic and concatenation operators C<+ - .>, but the perl4
1040variants of these operators actually bind tighter than C<+ - .>.
1041Thus, for:
1042
1043 %foo = 1..10;
1044 print keys %foo - 1
1045
1046 # perl4 prints: 4
1047 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1048
1049The perl4 behavior was probably more useful, if less consistent.
1050
6dbacca0
PP
1051=back
1052
1053=head2 General Regular Expression Traps using s///, etc.
1054
1055All types of RE traps.
1056
1057=over 5
1058
1059=item * Regular Expression
1060
1061C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
19799a22 1062interpolate $lhs but not $rhs. (And still does not match a literal
6dbacca0
PP
1063'$' in string)
1064
1065 $a=1;$b=2;
1066 $string = '1 2 $a $b';
1067 $string =~ s'$a'$b';
1068 print $string,"\n";
54310121 1069
6dbacca0
PP
1070 # perl4 prints: $b 2 $a $b
1071 # perl5 prints: 1 2 $a $b
1072
1073=item * Regular Expression
a0d0e21e
LW
1074
1075C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
1076regular expression. (Once the scope of a block is left for the sub, the
1077state of the searched string is lost)
1078
1079 $_ = "ababab";
1080 while(m/ab/g){
1081 &doit("blah");
1082 }
1083 sub doit{local($_) = shift; print "Got $_ "}
54310121 1084
6dbacca0
PP
1085 # perl4 prints: blah blah blah
1086 # perl5 prints: infinite loop blah...
1087
1088=item * Regular Expression
1089
68dc0745
PP
1090Currently, if you use the C<m//o> qualifier on a regular expression
1091within an anonymous sub, I<all> closures generated from that anonymous
1092sub will use the regular expression as it was compiled when it was used
1093the very first time in any such closure. For instance, if you say
1094
1095 sub build_match {
1096 my($left,$right) = @_;
1097 return sub { $_[0] =~ /$left stuff $right/o; };
1098 }
1099
1100build_match() will always return a sub which matches the contents of
19799a22 1101$left and $right as they were the I<first> time that build_match()
68dc0745
PP
1102was called, not as they are in the current call.
1103
1104This is probably a bug, and may change in future versions of Perl.
1105
1106=item * Regular Expression
1107
6dbacca0
PP
1108If no parentheses are used in a match, Perl4 sets C<$+> to
1109the whole match, just like C<$&>. Perl5 does not.
1110
1111 "abcdef" =~ /b.*e/;
1112 print "\$+ = $+\n";
54310121 1113
6dbacca0
PP
1114 # perl4 prints: bcde
1115 # perl5 prints:
1116
1117=item * Regular Expression
1118
1119substitution now returns the null string if it fails
1120
1121 $string = "test";
1122 $value = ($string =~ s/foo//);
1123 print $value, "\n";
54310121 1124
6dbacca0
PP
1125 # perl4 prints: 0
1126 # perl5 prints:
1127
1128Also see L<Numerical Traps> for another example of this new feature.
1129
1130=item * Regular Expression
1131
54310121
PP
1132C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1133backtick expansion
6dbacca0
PP
1134
1135 $string = "";
1136 $string =~ s`^`hostname`;
1137 print $string, "\n";
54310121 1138
6dbacca0
PP
1139 # perl4 prints: <the local hostname>
1140 # perl5 prints: hostname
1141
1142=item * Regular Expression
1143
1144Stricter parsing of variables used in regular expressions
1145
1146 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
54310121 1147
6dbacca0
PP
1148 # perl4: compiles w/o error
1149 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1150
1151an added component of this example, apparently from the same script, is
1152the actual value of the s'd string after the substitution.
1153C<[$opt]> is a character class in perl4 and an array subscript in perl5
1154
54310121 1155 $grpc = 'a';
6dbacca0
PP
1156 $opt = 'r';
1157 $_ = 'bar';
1158 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1159 print ;
54310121 1160
6dbacca0
PP
1161 # perl4 prints: foo
1162 # perl5 prints: foobar
1163
1164=item * Regular Expression
1165
1166Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1167repeatedly, like C</x/> or C<m!x!>.
1168
1169 $test = "once";
1170 sub match { $test =~ m?once?; }
1171 &match();
1172 if( &match() ) {
1173 # m?x? matches more then once
1174 print "perl4\n";
54310121 1175 } else {
6dbacca0 1176 # m?x? matches only once
54310121 1177 print "perl5\n";
6dbacca0 1178 }
54310121 1179
6dbacca0
PP
1180 # perl4 prints: perl4
1181 # perl5 prints: perl5
a0d0e21e 1182
a0d0e21e 1183
6dbacca0
PP
1184=back
1185
1186=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1187
6dbacca0
PP
1188The general group of Perl4-to-Perl5 traps having to do with
1189Signals, Sorting, and their related subroutines, as well as
1190general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1191
6dbacca0 1192=over 5
a0d0e21e 1193
6dbacca0 1194=item * (Signals)
a0d0e21e 1195
6dbacca0
PP
1196Barewords that used to look like strings to Perl will now look like subroutine
1197calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1198
6dbacca0
PP
1199 sub SeeYa { warn"Hasta la vista, baby!" }
1200 $SIG{'TERM'} = SeeYa;
1201 print "SIGTERM is now $SIG{'TERM'}\n";
54310121 1202
6dbacca0
PP
1203 # perl4 prints: SIGTERM is main'SeeYa
1204 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1205
6dbacca0 1206Use B<-w> to catch this one
a0d0e21e 1207
6dbacca0 1208=item * (Sort Subroutine)
a0d0e21e 1209
6dbacca0 1210reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1211
6dbacca0 1212 sub reverse{ print "yup "; $a <=> $b }
54310121
PP
1213 print sort reverse a,b,c;
1214
6dbacca0 1215 # perl4 prints: yup yup yup yup abc
54310121 1216 # perl5 prints: abc
a0d0e21e 1217
b996531f
PP
1218=item * warn() won't let you specify a filehandle.
1219
1220Although it _always_ printed to STDERR, warn() would let you specify a
1221filehandle in perl4. With perl5 it does not.
5e378fdf
PP
1222
1223 warn STDERR "Foo!";
1224
1225 # perl4 prints: Foo!
54310121 1226 # perl5 prints: String found where operator expected
5e378fdf 1227
6dbacca0 1228=back
a0d0e21e 1229
6dbacca0
PP
1230=head2 OS Traps
1231
1232=over 5
1233
1234=item * (SysV)
1235
54310121
PP
1236Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1237within the signal handler function, each time a signal was handled with
1238perl4. With perl5, the reset is now done correctly. Any code relying
6dbacca0
PP
1239on the handler _not_ being reset will have to be reworked.
1240
a6006777 1241Since version 5.002, Perl uses sigaction() under SysV.
6dbacca0
PP
1242
1243 sub gotit {
54310121
PP
1244 print "Got @_... ";
1245 }
6dbacca0 1246 $SIG{'INT'} = 'gotit';
54310121 1247
6dbacca0
PP
1248 $| = 1;
1249 $pid = fork;
1250 if ($pid) {
1251 kill('INT', $pid);
1252 sleep(1);
1253 kill('INT', $pid);
54310121 1254 } else {
6dbacca0 1255 while (1) {sleep(10);}
54310121
PP
1256 }
1257
6dbacca0
PP
1258 # perl4 (HPUX) prints: Got INT...
1259 # perl5 (HPUX) prints: Got INT... Got INT...
1260
1261=item * (SysV)
1262
54310121
PP
1263Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1264the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
6dbacca0
PP
1265for append, it is impossible to overwrite information already in
1266the file.
1267
1268 open(TEST,">>seek.test");
54310121 1269 $start = tell TEST ;
6dbacca0
PP
1270 foreach(1 .. 9){
1271 print TEST "$_ ";
1272 }
1273 $end = tell TEST ;
1274 seek(TEST,$start,0);
1275 print TEST "18 characters here";
54310121 1276
6dbacca0
PP
1277 # perl4 (solaris) seek.test has: 18 characters here
1278 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1279
a0d0e21e 1280
a0d0e21e 1281
6dbacca0 1282=back
a0d0e21e 1283
6dbacca0 1284=head2 Interpolation Traps
a0d0e21e 1285
8b0a4b75
PP
1286Perl4-to-Perl5 traps having to do with how things get interpolated
1287within certain expressions, statements, contexts, or whatever.
1288
6dbacca0 1289=over 5
a0d0e21e 1290
6dbacca0 1291=item * Interpolation
a0d0e21e 1292
6dbacca0
PP
1293@ now always interpolates an array in double-quotish strings.
1294
54310121
PP
1295 print "To: someone@somewhere.com\n";
1296
6dbacca0 1297 # perl4 prints: To:someone@somewhere.com
9607fc9c 1298 # perl5 errors : In string, @somewhere now must be written as \@somewhere
6dbacca0
PP
1299
1300=item * Interpolation
1301
6dbacca0
PP
1302Double-quoted strings may no longer end with an unescaped $ or @.
1303
1304 $foo = "foo$";
1305 $bar = "bar@";
1306 print "foo is $foo, bar is $bar\n";
54310121 1307
6dbacca0
PP
1308 # perl4 prints: foo is foo$, bar is bar@
1309 # perl5 errors: Final $ should be \$ or $name
1310
1311Note: perl5 DOES NOT error on the terminating @ in $bar
1312
1313=item * Interpolation
a0d0e21e 1314
8b0a4b75
PP
1315Perl now sometimes evaluates arbitrary expressions inside braces that occur
1316within double quotes (usually when the opening brace is preceded by C<$>
1317or C<@>).
1318
1319 @www = "buz";
1320 $foo = "foo";
1321 $bar = "bar";
1322 sub foo { return "bar" };
1323 print "|@{w.w.w}|${main'foo}|";
1324
1325 # perl4 prints: |@{w.w.w}|foo|
1326 # perl5 prints: |buz|bar|
1327
1328Note that you can C<use strict;> to ward off such trappiness under perl5.
1329
1330=item * Interpolation
1331
748a9306 1332The construct "this is $$x" used to interpolate the pid at that
19799a22 1333point, but now apparently tries to dereference $x. C<$$> by itself still
748a9306
LW
1334works fine, however.
1335
6dbacca0 1336 print "this is $$x\n";
748a9306 1337
6dbacca0
PP
1338 # perl4 prints: this is XXXx (XXX is the current pid)
1339 # perl5 prints: this is
1340
1341=item * Interpolation
1342
54310121
PP
1343Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1344C<$>'s to be protected in the specification of the hash name, or both curlies
6dbacca0
PP
1345to be protected. If both curlies are protected, the result will be compatible
1346with perl4 and perl5. This is a very common practice, and should be changed
1347to use the block form of C<eval{}> if possible.
c07a80fd 1348
6dbacca0
PP
1349 $hashname = "foobar";
1350 $key = "baz";
1351 $value = 1234;
1352 eval "\$$hashname{'$key'} = q|$value|";
1353 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1354
1355 # perl4 prints: Yup
1356 # perl5 prints: Nope
1357
1358Changing
1359
1360 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1361
1362to
1363
6dbacca0 1364 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1365
6dbacca0 1366causes the following result:
c07a80fd 1367
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PP
1368 # perl4 prints: Nope
1369 # perl5 prints: Yup
c07a80fd 1370
6dbacca0 1371or, changing to
a0d0e21e 1372
6dbacca0
PP
1373 eval "\$$hashname\{'$key'\} = q|$value|";
1374
1375causes the following result:
1376
1377 # perl4 prints: Yup
1378 # perl5 prints: Yup
1379 # and is compatible for both versions
1380
1381
1382=item * Interpolation
1383
1384perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1385
1386 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
54310121 1387
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PP
1388 # perl4 prints: This is not perl5
1389 # perl5 prints: This is perl5
1390
1391=item * Interpolation
1392
54310121 1393You also have to be careful about array references.
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PP
1394
1395 print "$foo{"
1396
1397 perl 4 prints: {
1398 perl 5 prints: syntax error
1399
1400=item * Interpolation
1401
1402Similarly, watch out for:
1403
1404 $foo = "array";
1405 print "\$$foo{bar}\n";
54310121 1406
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PP
1407 # perl4 prints: $array{bar}
1408 # perl5 prints: $
1409
1410Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1411happy just to expand $foo to "array" by itself. Watch out for this
1412especially in C<eval>'s.
1413
1414=item * Interpolation
1415
1416C<qq()> string passed to C<eval>
1417
1418 eval qq(
1419 foreach \$y (keys %\$x\) {
1420 \$count++;
1421 }
1422 );
54310121 1423
6dbacca0 1424 # perl4 runs this ok
54310121 1425 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1426
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PP
1427=back
1428
1429=head2 DBM Traps
1430
1431General DBM traps.
1432
1433=over 5
1434
1435=item * DBM
1436
1437Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1438may cause the same script, run under perl5, to fail. The build of perl5
1439must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1440to function properly without C<tie>'ing to an extension dbm implementation.
1441
1442 dbmopen (%dbm, "file", undef);
1443 print "ok\n";
1444
1445 # perl4 prints: ok
1446 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1447
1448
1449=item * DBM
1450
1451Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1452may cause the same script, run under perl5, to fail. The error generated
1453when exceeding the limit on the key/value size will cause perl5 to exit
1454immediately.
1455
1456 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1457 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1458 print "YUP\n";
1459
1460 # perl4 prints:
1461 dbm store returned -1, errno 28, key "trap" at - line 3.
1462 YUP
1463
1464 # perl5 prints:
1465 dbm store returned -1, errno 28, key "trap" at - line 3.
a0d0e21e
LW
1466
1467=back
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PP
1468
1469=head2 Unclassified Traps
1470
1471Everything else.
1472
84dc3c4d
PP
1473=over 5
1474
5db417f7 1475=item * C<require>/C<do> trap using returned value
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1476
1477If the file doit.pl has:
1478
1479 sub foo {
1480 $rc = do "./do.pl";
1481 return 8;
54310121 1482 }
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PP
1483 print &foo, "\n";
1484
1485And the do.pl file has the following single line:
1486
1487 return 3;
1488
1489Running doit.pl gives the following:
1490
1491 # perl 4 prints: 3 (aborts the subroutine early)
54310121 1492 # perl 5 prints: 8
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PP
1493
1494Same behavior if you replace C<do> with C<require>.
1495
5db417f7
TB
1496=item * C<split> on empty string with LIMIT specified
1497
1498 $string = '';
1499 @list = split(/foo/, $string, 2)
1500
1501Perl4 returns a one element list containing the empty string but Perl5
1502returns an empty list.
1503
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PP
1504=back
1505
54310121 1506As always, if any of these are ever officially declared as bugs,
6dbacca0
PP
1507they'll be fixed and removed.
1508