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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. ($0 is the name of the program
68executed.) See L<perlvar>.
69
70=item *
71
c47ff5f1 72$<I<digit>> does not refer to fields--it refers to substrings matched
8b0a4b75 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
c47ff5f1 106slightly context sensitive for operators like "/", "?", and ">".
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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
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175Comments begin with "#", not "/*".
176
177=item *
178
179You can't take the address of anything, although a similar operator
5f05dabc 180in Perl is the backslash, which creates a reference.
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181
182=item *
183
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184C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
185ends up in C<$0>.
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186
187=item *
188
189System calls such as link(), unlink(), rename(), etc. return nonzero for
190success, not 0.
191
192=item *
193
194Signal handlers deal with signal names, not numbers. Use C<kill -l>
195to find their names on your system.
196
197=back
198
199=head2 Sed Traps
200
201Seasoned B<sed> programmers should take note of the following:
202
203=over 4
204
205=item *
206
207Backreferences in substitutions use "$" rather than "\".
208
209=item *
210
211The pattern matching metacharacters "(", ")", and "|" do not have backslashes
212in front.
213
214=item *
215
216The range operator is C<...>, rather than comma.
217
218=back
219
220=head2 Shell Traps
221
222Sharp shell programmers should take note of the following:
223
224=over 4
225
226=item *
227
54310121 228The backtick operator does variable interpolation without regard to
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229the presence of single quotes in the command.
230
231=item *
232
54310121 233The backtick operator does no translation of the return value, unlike B<csh>.
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234
235=item *
236
237Shells (especially B<csh>) do several levels of substitution on each
5f05dabc 238command line. Perl does substitution in only certain constructs
54310121 239such as double quotes, backticks, angle brackets, and search patterns.
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240
241=item *
242
243Shells interpret scripts a little bit at a time. Perl compiles the
244entire program before executing it (except for C<BEGIN> blocks, which
245execute at compile time).
246
247=item *
248
249The arguments are available via @ARGV, not $1, $2, etc.
250
251=item *
252
253The environment is not automatically made available as separate scalar
254variables.
255
256=back
257
258=head2 Perl Traps
259
260Practicing Perl Programmers should take note of the following:
261
262=over 4
263
264=item *
265
266Remember that many operations behave differently in a list
267context than they do in a scalar one. See L<perldata> for details.
268
269=item *
270
68dc0745 271Avoid barewords if you can, especially all lowercase ones.
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272You can't tell by just looking at it whether a bareword is
273a function or a string. By using quotes on strings and
5f05dabc 274parentheses on function calls, you won't ever get them confused.
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275
276=item *
277
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278You cannot discern from mere inspection which builtins
279are unary operators (like chop() and chdir())
a0d0e21e 280and which are list operators (like print() and unlink()).
5f05dabc 281(User-defined subroutines can be B<only> list operators, never
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282unary ones.) See L<perlop>.
283
284=item *
285
748a9306 286People have a hard time remembering that some functions
a0d0e21e 287default to $_, or @ARGV, or whatever, but that others which
54310121 288you might expect to do not.
a0d0e21e 289
6dbacca0 290=item *
a0d0e21e 291
c47ff5f1 292The <FH> construct is not the name of the filehandle, it is a readline
5f05dabc 293operation on that handle. The data read is assigned to $_ only if the
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294file read is the sole condition in a while loop:
295
296 while (<FH>) { }
54310121 297 while (defined($_ = <FH>)) { }..
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298 <FH>; # data discarded!
299
6dbacca0 300=item *
748a9306 301
19799a22 302Remember not to use C<=> when you need C<=~>;
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303these two constructs are quite different:
304
305 $x = /foo/;
306 $x =~ /foo/;
307
308=item *
309
54310121 310The C<do {}> construct isn't a real loop that you can use
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311loop control on.
312
313=item *
314
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315Use C<my()> for local variables whenever you can get away with
316it (but see L<perlform> for where you can't).
317Using C<local()> actually gives a local value to a global
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318variable, which leaves you open to unforeseen side-effects
319of dynamic scoping.
320
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321=item *
322
323If you localize an exported variable in a module, its exported value will
324not change. The local name becomes an alias to a new value but the
325external name is still an alias for the original.
326
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327=back
328
5f05dabc 329=head2 Perl4 to Perl5 Traps
a0d0e21e 330
54310121 331Practicing Perl4 Programmers should take note of the following
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332Perl4-to-Perl5 specific traps.
333
334They're crudely ordered according to the following list:
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335
336=over 4
337
6dbacca0 338=item Discontinuance, Deprecation, and BugFix traps
a0d0e21e 339
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340Anything that's been fixed as a perl4 bug, removed as a perl4 feature
341or deprecated as a perl4 feature with the intent to encourage usage of
342some other perl5 feature.
a0d0e21e 343
6dbacca0 344=item Parsing Traps
748a9306 345
6dbacca0 346Traps that appear to stem from the new parser.
a0d0e21e 347
6dbacca0 348=item Numerical Traps
a0d0e21e 349
6dbacca0 350Traps having to do with numerical or mathematical operators.
a0d0e21e 351
6dbacca0 352=item General data type traps
a0d0e21e 353
6dbacca0 354Traps involving perl standard data types.
a0d0e21e 355
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356=item Context Traps - scalar, list contexts
357
358Traps related to context within lists, scalar statements/declarations.
359
360=item Precedence Traps
361
362Traps related to the precedence of parsing, evaluation, and execution of
363code.
364
365=item General Regular Expression Traps using s///, etc.
366
367Traps related to the use of pattern matching.
368
369=item Subroutine, Signal, Sorting Traps
370
371Traps related to the use of signals and signal handlers, general subroutines,
372and sorting, along with sorting subroutines.
373
374=item OS Traps
375
376OS-specific traps.
377
378=item DBM Traps
379
380Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
381
382=item Unclassified Traps
383
384Everything else.
385
386=back
387
388If you find an example of a conversion trap that is not listed here,
4375e838 389please submit it to <F<perlbug@perl.org>> for inclusion.
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390Also note that at least some of these can be caught with the
391C<use warnings> pragma or the B<-w> switch.
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392
393=head2 Discontinuance, Deprecation, and BugFix traps
394
395Anything that has been discontinued, deprecated, or fixed as
54310121 396a bug from perl4.
a0d0e21e 397
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398=over 4
399
54310121 400=item * Discontinuance
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401
402Symbols starting with "_" are no longer forced into package main, except
403for C<$_> itself (and C<@_>, etc.).
404
405 package test;
406 $_legacy = 1;
cb1a09d0 407
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408 package main;
409 print "\$_legacy is ",$_legacy,"\n";
54310121 410
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411 # perl4 prints: $_legacy is 1
412 # perl5 prints: $_legacy is
413
54310121 414=item * Deprecation
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415
416Double-colon is now a valid package separator in a variable name. Thus these
5f05dabc 417behave differently in perl4 vs. perl5, because the packages don't exist.
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418
419 $a=1;$b=2;$c=3;$var=4;
420 print "$a::$b::$c ";
cb1a09d0 421 print "$var::abc::xyz\n";
c47ff5f1 422
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423 # perl4 prints: 1::2::3 4::abc::xyz
424 # perl5 prints: 3
cb1a09d0 425
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426Given that C<::> is now the preferred package delimiter, it is debatable
427whether this should be classed as a bug or not.
428(The older package delimiter, ' ,is used here)
cb1a09d0 429
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430 $x = 10 ;
431 print "x=${'x}\n" ;
54310121 432
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433 # perl4 prints: x=10
434 # perl5 prints: Can't find string terminator "'" anywhere before EOF
a0d0e21e 435
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436You can avoid this problem, and remain compatible with perl4, if you
437always explicitly include the package name:
438
439 $x = 10 ;
440 print "x=${main'x}\n" ;
441
54310121 442Also see precedence traps, for parsing C<$:>.
a0d0e21e 443
6dbacca0 444=item * BugFix
a0d0e21e 445
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446The second and third arguments of C<splice()> are now evaluated in scalar
447context (as the Camel says) rather than list context.
a0d0e21e 448
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449 sub sub1{return(0,2) } # return a 2-element list
450 sub sub2{ return(1,2,3)} # return a 3-element list
54310121 451 @a1 = ("a","b","c","d","e");
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452 @a2 = splice(@a1,&sub1,&sub2);
453 print join(' ',@a2),"\n";
54310121 454
6dbacca0 455 # perl4 prints: a b
54310121 456 # perl5 prints: c d e
a0d0e21e 457
54310121 458=item * Discontinuance
a0d0e21e 459
6dbacca0 460You can't do a C<goto> into a block that is optimized away. Darn.
a0d0e21e 461
6dbacca0 462 goto marker1;
a0d0e21e 463
54310121 464 for(1){
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465 marker1:
466 print "Here I is!\n";
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467 }
468
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469 # perl4 prints: Here I is!
470 # perl5 dumps core (SEGV)
471
54310121 472=item * Discontinuance
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473
474It is no longer syntactically legal to use whitespace as the name
475of a variable, or as a delimiter for any kind of quote construct.
54310121 476Double darn.
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477
478 $a = ("foo bar");
479 $b = q baz ;
480 print "a is $a, b is $b\n";
54310121 481
6dbacca0 482 # perl4 prints: a is foo bar, b is baz
54310121 483 # perl5 errors: Bareword found where operator expected
5e378fdf 484
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485=item * Discontinuance
486
487The archaic while/if BLOCK BLOCK syntax is no longer supported.
488
489 if { 1 } {
490 print "True!";
491 }
492 else {
493 print "False!";
494 }
54310121 495
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496 # perl4 prints: True!
497 # perl5 errors: syntax error at test.pl line 1, near "if {"
498
499=item * BugFix
500
501The C<**> operator now binds more tightly than unary minus.
502It was documented to work this way before, but didn't.
503
504 print -4**2,"\n";
54310121 505
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506 # perl4 prints: 16
507 # perl5 prints: -16
508
54310121 509=item * Discontinuance
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510
511The meaning of C<foreach{}> has changed slightly when it is iterating over a
512list which is not an array. This used to assign the list to a
513temporary array, but no longer does so (for efficiency). This means
514that you'll now be iterating over the actual values, not over copies of
515the values. Modifications to the loop variable can change the original
516values.
517
518 @list = ('ab','abc','bcd','def');
519 foreach $var (grep(/ab/,@list)){
520 $var = 1;
521 }
522 print (join(':',@list));
54310121 523
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524 # perl4 prints: ab:abc:bcd:def
525 # perl5 prints: 1:1:bcd:def
526
527To retain Perl4 semantics you need to assign your list
54310121 528explicitly to a temporary array and then iterate over that. For
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529example, you might need to change
530
531 foreach $var (grep(/ab/,@list)){
532
533to
534
535 foreach $var (@tmp = grep(/ab/,@list)){
536
537Otherwise changing $var will clobber the values of @list. (This most often
538happens when you use C<$_> for the loop variable, and call subroutines in
539the loop that don't properly localize C<$_>.)
540
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541=item * Discontinuance
542
543C<split> with no arguments now behaves like C<split ' '> (which doesn't
544return an initial null field if $_ starts with whitespace), it used to
545behave like C<split /\s+/> (which does).
546
547 $_ = ' hi mom';
548 print join(':', split);
549
550 # perl4 prints: :hi:mom
551 # perl5 prints: hi:mom
552
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553=item * BugFix
554
9607fc9c 555Perl 4 would ignore any text which was attached to an B<-e> switch,
55497cff 556always taking the code snippet from the following arg. Additionally, it
9607fc9c 557would silently accept an B<-e> switch without a following arg. Both of
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558these behaviors have been fixed.
559
560 perl -e'print "attached to -e"' 'print "separate arg"'
54310121 561
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562 # perl4 prints: separate arg
563 # perl5 prints: attached to -e
54310121 564
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565 perl -e
566
567 # perl4 prints:
568 # perl5 dies: No code specified for -e.
569
570=item * Discontinuance
571
572In Perl 4 the return value of C<push> was undocumented, but it was
573actually the last value being pushed onto the target list. In Perl 5
574the return value of C<push> is documented, but has changed, it is the
575number of elements in the resulting list.
576
577 @x = ('existing');
578 print push(@x, 'first new', 'second new');
54310121 579
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580 # perl4 prints: second new
581 # perl5 prints: 3
582
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583=item * Deprecation
584
585Some error messages will be different.
586
54310121 587=item * Discontinuance
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588
589Some bugs may have been inadvertently removed. :-)
590
591=back
592
593=head2 Parsing Traps
594
595Perl4-to-Perl5 traps from having to do with parsing.
596
597=over 4
598
599=item * Parsing
600
601Note the space between . and =
602
603 $string . = "more string";
604 print $string;
54310121 605
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606 # perl4 prints: more string
607 # perl5 prints: syntax error at - line 1, near ". ="
608
609=item * Parsing
610
611Better parsing in perl 5
612
613 sub foo {}
614 &foo
615 print("hello, world\n");
54310121 616
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617 # perl4 prints: hello, world
618 # perl5 prints: syntax error
619
620=item * Parsing
621
622"if it looks like a function, it is a function" rule.
623
624 print
625 ($foo == 1) ? "is one\n" : "is zero\n";
54310121 626
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627 # perl4 prints: is zero
628 # perl5 warns: "Useless use of a constant in void context" if using -w
629
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630=item * Parsing
631
632String interpolation of the C<$#array> construct differs when braces
633are to used around the name.
634
635 @ = (1..3);
636 print "${#a}";
637
638 # perl4 prints: 2
639 # perl5 fails with syntax error
640
641 @ = (1..3);
642 print "$#{a}";
643
644 # perl4 prints: {a}
645 # perl5 prints: 2
646
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647=back
648
649=head2 Numerical Traps
650
651Perl4-to-Perl5 traps having to do with numerical operators,
652operands, or output from same.
653
654=over 5
655
656=item * Numerical
657
658Formatted output and significant digits
659
660 print 7.373504 - 0, "\n";
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661 printf "%20.18f\n", 7.373504 - 0;
662
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663 # Perl4 prints:
664 7.375039999999996141
665 7.37503999999999614
54310121 666
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667 # Perl5 prints:
668 7.373504
669 7.37503999999999614
670
671=item * Numerical
672
5f05dabc 673This specific item has been deleted. It demonstrated how the auto-increment
5e378fdf 674operator would not catch when a number went over the signed int limit. Fixed
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675in version 5.003_04. But always be wary when using large integers.
676If in doubt:
6dbacca0 677
5e378fdf 678 use Math::BigInt;
6dbacca0 679
54310121 680=item * Numerical
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681
682Assignment of return values from numeric equality tests
683does not work in perl5 when the test evaluates to false (0).
684Logical tests now return an null, instead of 0
a6006777 685
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686 $p = ($test == 1);
687 print $p,"\n";
a6006777 688
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689 # perl4 prints: 0
690 # perl5 prints:
691
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692Also see L<"General Regular Expression Traps using s///, etc.">
693for another example of this new feature...
6dbacca0 694
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695=item * Bitwise string ops
696
697When bitwise operators which can operate upon either numbers or
698strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
699treat the operands as bitstrings so long as the program contained a call
700to the C<vec()> function. perl5 treats the string operands as bitstrings.
701(See L<perlop/Bitwise String Operators> for more details.)
702
703 $fred = "10";
704 $barney = "12";
705 $betty = $fred & $barney;
706 print "$betty\n";
707 # Uncomment the next line to change perl4's behavior
708 # ($dummy) = vec("dummy", 0, 0);
709
710 # Perl4 prints:
711 8
712
713 # Perl5 prints:
714 10
715
716 # If vec() is used anywhere in the program, both print:
717 10
718
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719=back
720
721=head2 General data type traps
722
723Perl4-to-Perl5 traps involving most data-types, and their usage
724within certain expressions and/or context.
725
726=over 5
727
728=item * (Arrays)
729
730Negative array subscripts now count from the end of the array.
731
732 @a = (1, 2, 3, 4, 5);
733 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
54310121 734
6dbacca0
PP
735 # perl4 prints: The third element of the array is 4 also expressed as
736 # perl5 prints: The third element of the array is 4 also expressed as 4
737
738=item * (Arrays)
739
740Setting C<$#array> lower now discards array elements, and makes them
741impossible to recover.
742
54310121 743 @a = (a,b,c,d,e);
6dbacca0 744 print "Before: ",join('',@a);
54310121 745 $#a =1;
6dbacca0
PP
746 print ", After: ",join('',@a);
747 $#a =3;
748 print ", Recovered: ",join('',@a),"\n";
54310121 749
6dbacca0
PP
750 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
751 # perl5 prints: Before: abcde, After: ab, Recovered: ab
752
753=item * (Hashes)
754
755Hashes get defined before use
756
54310121 757 local($s,@a,%h);
6dbacca0
PP
758 die "scalar \$s defined" if defined($s);
759 die "array \@a defined" if defined(@a);
760 die "hash \%h defined" if defined(%h);
54310121 761
6dbacca0
PP
762 # perl4 prints:
763 # perl5 dies: hash %h defined
764
475342a6
GS
765Perl will now generate a warning when it sees defined(@a) and
766defined(%h).
767
6dbacca0
PP
768=item * (Globs)
769
770glob assignment from variable to variable will fail if the assigned
771variable is localized subsequent to the assignment
772
773 @a = ("This is Perl 4");
774 *b = *a;
775 local(@a);
776 print @b,"\n";
54310121 777
6dbacca0
PP
778 # perl4 prints: This is Perl 4
779 # perl5 prints:
54310121 780
a3cb178b 781=item * (Globs)
54310121 782
a3cb178b
GS
783Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
784it undefines the associated scalar (but may have other side effects
785including SEGVs).
5e378fdf 786
6dbacca0
PP
787=item * (Scalar String)
788
789Changes in unary negation (of strings)
790This change effects both the return value and what it
791does to auto(magic)increment.
792
793 $x = "aaa";
794 print ++$x," : ";
795 print -$x," : ";
796 print ++$x,"\n";
54310121 797
6dbacca0
PP
798 # perl4 prints: aab : -0 : 1
799 # perl5 prints: aab : -aab : aac
800
801=item * (Constants)
802
803perl 4 lets you modify constants:
804
805 $foo = "x";
806 &mod($foo);
807 for ($x = 0; $x < 3; $x++) {
808 &mod("a");
809 }
810 sub mod {
811 print "before: $_[0]";
812 $_[0] = "m";
813 print " after: $_[0]\n";
814 }
54310121 815
6dbacca0
PP
816 # perl4:
817 # before: x after: m
818 # before: a after: m
819 # before: m after: m
820 # before: m after: m
54310121 821
6dbacca0
PP
822 # Perl5:
823 # before: x after: m
824 # Modification of a read-only value attempted at foo.pl line 12.
825 # before: a
826
827=item * (Scalars)
828
829The behavior is slightly different for:
830
831 print "$x", defined $x
54310121 832
6dbacca0
PP
833 # perl 4: 1
834 # perl 5: <no output, $x is not called into existence>
835
836=item * (Variable Suicide)
837
838Variable suicide behavior is more consistent under Perl 5.
aa689395 839Perl5 exhibits the same behavior for hashes and scalars,
5f05dabc 840that perl4 exhibits for only scalars.
6dbacca0
PP
841
842 $aGlobal{ "aKey" } = "global value";
843 print "MAIN:", $aGlobal{"aKey"}, "\n";
844 $GlobalLevel = 0;
845 &test( *aGlobal );
846
847 sub test {
848 local( *theArgument ) = @_;
849 local( %aNewLocal ); # perl 4 != 5.001l,m
54310121 850 $aNewLocal{"aKey"} = "this should never appear";
6dbacca0
PP
851 print "SUB: ", $theArgument{"aKey"}, "\n";
852 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
853 $GlobalLevel++;
854 if( $GlobalLevel<4 ) {
855 &test( *aNewLocal );
856 }
857 }
54310121 858
6dbacca0
PP
859 # Perl4:
860 # MAIN:global value
861 # SUB: global value
862 # SUB: level 0
863 # SUB: level 1
864 # SUB: level 2
54310121 865
6dbacca0
PP
866 # Perl5:
867 # MAIN:global value
868 # SUB: global value
869 # SUB: this should never appear
870 # SUB: this should never appear
871 # SUB: this should never appear
872
84dc3c4d 873=back
6dbacca0
PP
874
875=head2 Context Traps - scalar, list contexts
876
877=over 5
878
879=item * (list context)
880
881The elements of argument lists for formats are now evaluated in list
882context. This means you can interpolate list values now.
883
884 @fmt = ("foo","bar","baz");
885 format STDOUT=
886 @<<<<< @||||| @>>>>>
887 @fmt;
888 .
54310121
PP
889 write;
890
6dbacca0
PP
891 # perl4 errors: Please use commas to separate fields in file
892 # perl5 prints: foo bar baz
893
894=item * (scalar context)
895
54310121
PP
896The C<caller()> function now returns a false value in a scalar context
897if there is no caller. This lets library files determine if they're
6dbacca0
PP
898being required.
899
900 caller() ? (print "You rang?\n") : (print "Got a 0\n");
54310121 901
6dbacca0
PP
902 # perl4 errors: There is no caller
903 # perl5 prints: Got a 0
5e378fdf 904
6dbacca0
PP
905=item * (scalar context)
906
907The comma operator in a scalar context is now guaranteed to give a
908scalar context to its arguments.
909
910 @y= ('a','b','c');
911 $x = (1, 2, @y);
912 print "x = $x\n";
54310121 913
6dbacca0
PP
914 # Perl4 prints: x = c # Thinks list context interpolates list
915 # Perl5 prints: x = 3 # Knows scalar uses length of list
916
917=item * (list, builtin)
918
919C<sprintf()> funkiness (array argument converted to scalar array count)
920This test could be added to t/op/sprintf.t
921
922 @z = ('%s%s', 'foo', 'bar');
923 $x = sprintf(@z);
924 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
54310121 925
6dbacca0
PP
926 # perl4 prints: ok 2
927 # perl5 prints: not ok 2
928
929C<printf()> works fine, though:
930
931 printf STDOUT (@z);
54310121
PP
932 print "\n";
933
6dbacca0
PP
934 # perl4 prints: foobar
935 # perl5 prints: foobar
936
937Probably a bug.
938
939=back
940
941=head2 Precedence Traps
942
943Perl4-to-Perl5 traps involving precedence order.
944
f4b17341
GS
945Perl 4 has almost the same precedence rules as Perl 5 for the operators
946that they both have. Perl 4 however, seems to have had some
947inconsistencies that made the behavior differ from what was documented.
948
84dc3c4d
PP
949=over 5
950
5e378fdf
PP
951=item * Precedence
952
8dbef698
JM
953LHS vs. RHS of any assignment operator. LHS is evaluated first
954in perl4, second in perl5; this can affect the relationship
955between side-effects in sub-expressions.
5e378fdf
PP
956
957 @arr = ( 'left', 'right' );
958 $a{shift @arr} = shift @arr;
959 print join( ' ', keys %a );
960
961 # perl4 prints: left
962 # perl5 prints: right
963
964=item * Precedence
6dbacca0
PP
965
966These are now semantic errors because of precedence:
967
968 @list = (1,2,3,4,5);
969 %map = ("a",1,"b",2,"c",3,"d",4);
970 $n = shift @list + 2; # first item in list plus 2
971 print "n is $n, ";
972 $m = keys %map + 2; # number of items in hash plus 2
973 print "m is $m\n";
54310121 974
6dbacca0
PP
975 # perl4 prints: n is 3, m is 6
976 # perl5 errors and fails to compile
977
978=item * Precedence
a0d0e21e 979
4633a7c4
LW
980The precedence of assignment operators is now the same as the precedence
981of assignment. Perl 4 mistakenly gave them the precedence of the associated
982operator. So you now must parenthesize them in expressions like
983
984 /foo/ ? ($a += 2) : ($a -= 2);
a6006777 985
4633a7c4
LW
986Otherwise
987
6dbacca0 988 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
989
990would be erroneously parsed as
991
992 (/foo/ ? $a += 2 : $a) -= 2;
993
994On the other hand,
995
54310121 996 $a += /foo/ ? 1 : 2;
4633a7c4
LW
997
998now works as a C programmer would expect.
999
6dbacca0 1000=item * Precedence
4633a7c4 1001
6dbacca0 1002 open FOO || die;
a0d0e21e 1003
5f05dabc
PP
1004is now incorrect. You need parentheses around the filehandle.
1005Otherwise, perl5 leaves the statement as its default precedence:
a0d0e21e 1006
6dbacca0 1007 open(FOO || die);
54310121 1008
6dbacca0
PP
1009 # perl4 opens or dies
1010 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 1011
6dbacca0 1012=item * Precedence
a0d0e21e 1013
6dbacca0
PP
1014perl4 gives the special variable, C<$:> precedence, where perl5
1015treats C<$::> as main C<package>
a0d0e21e 1016
6dbacca0 1017 $a = "x"; print "$::a";
54310121 1018
6dbacca0
PP
1019 # perl 4 prints: -:a
1020 # perl 5 prints: x
5e378fdf 1021
6dbacca0 1022=item * Precedence
a0d0e21e 1023
f4b17341
GS
1024perl4 had buggy precedence for the file test operators vis-a-vis
1025the assignment operators. Thus, although the precedence table
1026for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1027C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1028In perl5, the precedence is as documented.
54310121
PP
1029
1030 -e $foo .= "q"
a0d0e21e 1031
6dbacca0
PP
1032 # perl4 prints: no output
1033 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 1034
f4b17341
GS
1035=item * Precedence
1036
1037In perl4, keys(), each() and values() were special high-precedence operators
1038that operated on a single hash, but in perl5, they are regular named unary
1039operators. As documented, named unary operators have lower precedence
1040than the arithmetic and concatenation operators C<+ - .>, but the perl4
1041variants of these operators actually bind tighter than C<+ - .>.
1042Thus, for:
1043
1044 %foo = 1..10;
1045 print keys %foo - 1
1046
1047 # perl4 prints: 4
1048 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1049
1050The perl4 behavior was probably more useful, if less consistent.
1051
6dbacca0
PP
1052=back
1053
1054=head2 General Regular Expression Traps using s///, etc.
1055
1056All types of RE traps.
1057
1058=over 5
1059
1060=item * Regular Expression
1061
1062C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
19799a22 1063interpolate $lhs but not $rhs. (And still does not match a literal
6dbacca0
PP
1064'$' in string)
1065
1066 $a=1;$b=2;
1067 $string = '1 2 $a $b';
1068 $string =~ s'$a'$b';
1069 print $string,"\n";
54310121 1070
6dbacca0
PP
1071 # perl4 prints: $b 2 $a $b
1072 # perl5 prints: 1 2 $a $b
1073
1074=item * Regular Expression
a0d0e21e
LW
1075
1076C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
1077regular expression. (Once the scope of a block is left for the sub, the
1078state of the searched string is lost)
1079
1080 $_ = "ababab";
1081 while(m/ab/g){
1082 &doit("blah");
1083 }
1084 sub doit{local($_) = shift; print "Got $_ "}
54310121 1085
6dbacca0
PP
1086 # perl4 prints: blah blah blah
1087 # perl5 prints: infinite loop blah...
1088
1089=item * Regular Expression
1090
68dc0745
PP
1091Currently, if you use the C<m//o> qualifier on a regular expression
1092within an anonymous sub, I<all> closures generated from that anonymous
1093sub will use the regular expression as it was compiled when it was used
1094the very first time in any such closure. For instance, if you say
1095
1096 sub build_match {
1097 my($left,$right) = @_;
1098 return sub { $_[0] =~ /$left stuff $right/o; };
1099 }
1100
1101build_match() will always return a sub which matches the contents of
19799a22 1102$left and $right as they were the I<first> time that build_match()
68dc0745
PP
1103was called, not as they are in the current call.
1104
1105This is probably a bug, and may change in future versions of Perl.
1106
1107=item * Regular Expression
1108
6dbacca0
PP
1109If no parentheses are used in a match, Perl4 sets C<$+> to
1110the whole match, just like C<$&>. Perl5 does not.
1111
1112 "abcdef" =~ /b.*e/;
1113 print "\$+ = $+\n";
54310121 1114
6dbacca0
PP
1115 # perl4 prints: bcde
1116 # perl5 prints:
1117
1118=item * Regular Expression
1119
1120substitution now returns the null string if it fails
1121
1122 $string = "test";
1123 $value = ($string =~ s/foo//);
1124 print $value, "\n";
54310121 1125
6dbacca0
PP
1126 # perl4 prints: 0
1127 # perl5 prints:
1128
1129Also see L<Numerical Traps> for another example of this new feature.
1130
1131=item * Regular Expression
1132
54310121
PP
1133C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1134backtick expansion
6dbacca0
PP
1135
1136 $string = "";
1137 $string =~ s`^`hostname`;
1138 print $string, "\n";
54310121 1139
6dbacca0
PP
1140 # perl4 prints: <the local hostname>
1141 # perl5 prints: hostname
1142
1143=item * Regular Expression
1144
1145Stricter parsing of variables used in regular expressions
1146
1147 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
54310121 1148
6dbacca0
PP
1149 # perl4: compiles w/o error
1150 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1151
1152an added component of this example, apparently from the same script, is
1153the actual value of the s'd string after the substitution.
1154C<[$opt]> is a character class in perl4 and an array subscript in perl5
1155
54310121 1156 $grpc = 'a';
6dbacca0
PP
1157 $opt = 'r';
1158 $_ = 'bar';
1159 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1160 print ;
54310121 1161
6dbacca0
PP
1162 # perl4 prints: foo
1163 # perl5 prints: foobar
1164
1165=item * Regular Expression
1166
1167Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1168repeatedly, like C</x/> or C<m!x!>.
1169
1170 $test = "once";
1171 sub match { $test =~ m?once?; }
1172 &match();
1173 if( &match() ) {
1174 # m?x? matches more then once
1175 print "perl4\n";
54310121 1176 } else {
6dbacca0 1177 # m?x? matches only once
54310121 1178 print "perl5\n";
6dbacca0 1179 }
54310121 1180
6dbacca0
PP
1181 # perl4 prints: perl4
1182 # perl5 prints: perl5
a0d0e21e 1183
a0d0e21e 1184
6dbacca0
PP
1185=back
1186
1187=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1188
6dbacca0
PP
1189The general group of Perl4-to-Perl5 traps having to do with
1190Signals, Sorting, and their related subroutines, as well as
1191general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1192
6dbacca0 1193=over 5
a0d0e21e 1194
6dbacca0 1195=item * (Signals)
a0d0e21e 1196
6dbacca0
PP
1197Barewords that used to look like strings to Perl will now look like subroutine
1198calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1199
6dbacca0
PP
1200 sub SeeYa { warn"Hasta la vista, baby!" }
1201 $SIG{'TERM'} = SeeYa;
1202 print "SIGTERM is now $SIG{'TERM'}\n";
54310121 1203
6dbacca0
PP
1204 # perl4 prints: SIGTERM is main'SeeYa
1205 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1206
6dbacca0 1207Use B<-w> to catch this one
a0d0e21e 1208
6dbacca0 1209=item * (Sort Subroutine)
a0d0e21e 1210
6dbacca0 1211reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1212
6dbacca0 1213 sub reverse{ print "yup "; $a <=> $b }
54310121
PP
1214 print sort reverse a,b,c;
1215
6dbacca0 1216 # perl4 prints: yup yup yup yup abc
54310121 1217 # perl5 prints: abc
a0d0e21e 1218
b996531f
PP
1219=item * warn() won't let you specify a filehandle.
1220
1221Although it _always_ printed to STDERR, warn() would let you specify a
1222filehandle in perl4. With perl5 it does not.
5e378fdf
PP
1223
1224 warn STDERR "Foo!";
1225
1226 # perl4 prints: Foo!
54310121 1227 # perl5 prints: String found where operator expected
5e378fdf 1228
6dbacca0 1229=back
a0d0e21e 1230
6dbacca0
PP
1231=head2 OS Traps
1232
1233=over 5
1234
1235=item * (SysV)
1236
54310121
PP
1237Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1238within the signal handler function, each time a signal was handled with
1239perl4. With perl5, the reset is now done correctly. Any code relying
6dbacca0
PP
1240on the handler _not_ being reset will have to be reworked.
1241
a6006777 1242Since version 5.002, Perl uses sigaction() under SysV.
6dbacca0
PP
1243
1244 sub gotit {
54310121
PP
1245 print "Got @_... ";
1246 }
6dbacca0 1247 $SIG{'INT'} = 'gotit';
54310121 1248
6dbacca0
PP
1249 $| = 1;
1250 $pid = fork;
1251 if ($pid) {
1252 kill('INT', $pid);
1253 sleep(1);
1254 kill('INT', $pid);
54310121 1255 } else {
6dbacca0 1256 while (1) {sleep(10);}
54310121
PP
1257 }
1258
6dbacca0
PP
1259 # perl4 (HPUX) prints: Got INT...
1260 # perl5 (HPUX) prints: Got INT... Got INT...
1261
1262=item * (SysV)
1263
c47ff5f1 1264Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
54310121 1265the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
6dbacca0
PP
1266for append, it is impossible to overwrite information already in
1267the file.
1268
1269 open(TEST,">>seek.test");
54310121 1270 $start = tell TEST ;
6dbacca0
PP
1271 foreach(1 .. 9){
1272 print TEST "$_ ";
1273 }
1274 $end = tell TEST ;
1275 seek(TEST,$start,0);
1276 print TEST "18 characters here";
54310121 1277
6dbacca0
PP
1278 # perl4 (solaris) seek.test has: 18 characters here
1279 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1280
a0d0e21e 1281
a0d0e21e 1282
6dbacca0 1283=back
a0d0e21e 1284
6dbacca0 1285=head2 Interpolation Traps
a0d0e21e 1286
8b0a4b75
PP
1287Perl4-to-Perl5 traps having to do with how things get interpolated
1288within certain expressions, statements, contexts, or whatever.
1289
6dbacca0 1290=over 5
a0d0e21e 1291
6dbacca0 1292=item * Interpolation
a0d0e21e 1293
6dbacca0
PP
1294@ now always interpolates an array in double-quotish strings.
1295
54310121
PP
1296 print "To: someone@somewhere.com\n";
1297
6dbacca0 1298 # perl4 prints: To:someone@somewhere.com
9607fc9c 1299 # perl5 errors : In string, @somewhere now must be written as \@somewhere
6dbacca0
PP
1300
1301=item * Interpolation
1302
6dbacca0
PP
1303Double-quoted strings may no longer end with an unescaped $ or @.
1304
1305 $foo = "foo$";
1306 $bar = "bar@";
1307 print "foo is $foo, bar is $bar\n";
54310121 1308
6dbacca0
PP
1309 # perl4 prints: foo is foo$, bar is bar@
1310 # perl5 errors: Final $ should be \$ or $name
1311
1312Note: perl5 DOES NOT error on the terminating @ in $bar
1313
1314=item * Interpolation
a0d0e21e 1315
8b0a4b75
PP
1316Perl now sometimes evaluates arbitrary expressions inside braces that occur
1317within double quotes (usually when the opening brace is preceded by C<$>
1318or C<@>).
1319
1320 @www = "buz";
1321 $foo = "foo";
1322 $bar = "bar";
1323 sub foo { return "bar" };
1324 print "|@{w.w.w}|${main'foo}|";
1325
1326 # perl4 prints: |@{w.w.w}|foo|
1327 # perl5 prints: |buz|bar|
1328
1329Note that you can C<use strict;> to ward off such trappiness under perl5.
1330
1331=item * Interpolation
1332
748a9306 1333The construct "this is $$x" used to interpolate the pid at that
19799a22 1334point, but now apparently tries to dereference $x. C<$$> by itself still
748a9306
LW
1335works fine, however.
1336
6dbacca0 1337 print "this is $$x\n";
748a9306 1338
6dbacca0
PP
1339 # perl4 prints: this is XXXx (XXX is the current pid)
1340 # perl5 prints: this is
1341
1342=item * Interpolation
1343
54310121
PP
1344Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1345C<$>'s to be protected in the specification of the hash name, or both curlies
6dbacca0
PP
1346to be protected. If both curlies are protected, the result will be compatible
1347with perl4 and perl5. This is a very common practice, and should be changed
1348to use the block form of C<eval{}> if possible.
c07a80fd 1349
6dbacca0
PP
1350 $hashname = "foobar";
1351 $key = "baz";
1352 $value = 1234;
1353 eval "\$$hashname{'$key'} = q|$value|";
1354 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1355
1356 # perl4 prints: Yup
1357 # perl5 prints: Nope
1358
1359Changing
1360
1361 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1362
1363to
1364
6dbacca0 1365 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1366
6dbacca0 1367causes the following result:
c07a80fd 1368
6dbacca0
PP
1369 # perl4 prints: Nope
1370 # perl5 prints: Yup
c07a80fd 1371
6dbacca0 1372or, changing to
a0d0e21e 1373
6dbacca0
PP
1374 eval "\$$hashname\{'$key'\} = q|$value|";
1375
1376causes the following result:
1377
1378 # perl4 prints: Yup
1379 # perl5 prints: Yup
1380 # and is compatible for both versions
1381
1382
1383=item * Interpolation
1384
1385perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1386
1387 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
54310121 1388
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PP
1389 # perl4 prints: This is not perl5
1390 # perl5 prints: This is perl5
1391
1392=item * Interpolation
1393
54310121 1394You also have to be careful about array references.
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PP
1395
1396 print "$foo{"
1397
1398 perl 4 prints: {
1399 perl 5 prints: syntax error
1400
1401=item * Interpolation
1402
1403Similarly, watch out for:
1404
1405 $foo = "array";
1406 print "\$$foo{bar}\n";
54310121 1407
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PP
1408 # perl4 prints: $array{bar}
1409 # perl5 prints: $
1410
1411Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1412happy just to expand $foo to "array" by itself. Watch out for this
1413especially in C<eval>'s.
1414
1415=item * Interpolation
1416
1417C<qq()> string passed to C<eval>
1418
1419 eval qq(
1420 foreach \$y (keys %\$x\) {
1421 \$count++;
1422 }
1423 );
54310121 1424
6dbacca0 1425 # perl4 runs this ok
54310121 1426 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1427
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PP
1428=back
1429
1430=head2 DBM Traps
1431
1432General DBM traps.
1433
1434=over 5
1435
1436=item * DBM
1437
1438Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1439may cause the same script, run under perl5, to fail. The build of perl5
1440must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1441to function properly without C<tie>'ing to an extension dbm implementation.
1442
1443 dbmopen (%dbm, "file", undef);
1444 print "ok\n";
1445
1446 # perl4 prints: ok
1447 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1448
1449
1450=item * DBM
1451
1452Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1453may cause the same script, run under perl5, to fail. The error generated
1454when exceeding the limit on the key/value size will cause perl5 to exit
1455immediately.
1456
1457 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1458 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1459 print "YUP\n";
1460
1461 # perl4 prints:
1462 dbm store returned -1, errno 28, key "trap" at - line 3.
1463 YUP
1464
1465 # perl5 prints:
1466 dbm store returned -1, errno 28, key "trap" at - line 3.
a0d0e21e
LW
1467
1468=back
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PP
1469
1470=head2 Unclassified Traps
1471
1472Everything else.
1473
84dc3c4d
PP
1474=over 5
1475
5db417f7 1476=item * C<require>/C<do> trap using returned value
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PP
1477
1478If the file doit.pl has:
1479
1480 sub foo {
1481 $rc = do "./do.pl";
1482 return 8;
54310121 1483 }
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PP
1484 print &foo, "\n";
1485
1486And the do.pl file has the following single line:
1487
1488 return 3;
1489
1490Running doit.pl gives the following:
1491
1492 # perl 4 prints: 3 (aborts the subroutine early)
54310121 1493 # perl 5 prints: 8
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PP
1494
1495Same behavior if you replace C<do> with C<require>.
1496
5db417f7
TB
1497=item * C<split> on empty string with LIMIT specified
1498
1499 $string = '';
1500 @list = split(/foo/, $string, 2)
1501
1502Perl4 returns a one element list containing the empty string but Perl5
1503returns an empty list.
1504
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PP
1505=back
1506
54310121 1507As always, if any of these are ever officially declared as bugs,
6dbacca0
PP
1508they'll be fixed and removed.
1509