While considering this bit of code again: 
perl -pe'}{$_=$.' filename

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and the implications of the implied continue, I wondered outloud about unobvious pathologies in general. Consider, for your amusement, what this does: 
for (qw(Whom the bell tolls)) {}
continue { print "$_\n"; next }

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[We could argue that this example should produce a warning, but this post is really about enjoying the quirks, and not about fixing them.]

What pathologies twist the corners of your mouth?

-QM
--
Quantum Mechanics: The dreams stuff is made of

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Re: Unobvious Pathological Code Snippets
by gaal (Parson) on Mar 28, 2006 at 17:12 UTC
    A recent favorite, noted by Dominus, is that your example could be rewritten as: 

    for $t qw(Whom the bell tolls) {}            # huh? for qw() ?
continue { print "$t\n"; next }

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    Update: fixed code, frodo72++

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        It's not like this is intended behavior :)

        My other favorite is even sicker and more useful. Credit this time to audreyt++ who checked this into pugs with the commit message

        do not mention $@% in the Makefile for portability

        We had a one-liner that was breaking differently on different platforms. Insane desugaring to the rescue! 

        -       \$(PERL) -MFile::Spec -e "my (undef, \$\$dir, \$\$file) = File
+::Spec->splitpath(shift); chdir(\$\$dir); system(q+$hsc2hs $hsc2hs_fl
+ags + . \$\$file);" \$<
+       \$(PERL) -MFile::Spec -e "sub p () { File::Spec->splitpath(ARG
+V->[0]) }; chdir((p)[1]); system(q($hsc2hs), qw($hsc2hs_flags), (p)[2
+]);" \$<

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        (The thing to note here is ARGV->[n].)
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      Am I missing something? With perl 5.8.8 in linux I get an error:
      poletti@PolettiX:~/sviluppo/perl$ perl pathological.pl
Missing $ on loop variable at pathological.pl line 1.
poletti@PolettiX:~/sviluppo/perl$ 
poletti@PolettiX:~/sviluppo/perl$ 
poletti@PolettiX:~/sviluppo/perl$ cat pathological.pl
for qw(Whom the bell tolls) {}            # huh? for qw() ?
continue { print "$_\n"; next }

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      while the OP's example...

      Flavio
      perl -ple'$_=reverse' <<<ti.xittelop@oivalf

      Don't fool yourself.
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        Oops, ()-less qw requires a loop variable. Fixed.
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Re: Unobvious Pathological Code Snippets
by BrowserUk (Patriarch) on Mar 28, 2006 at 18:11 UTC

    This, (attributable to Juerd) is kinda cute, and a little surprising: 

    my @a = '01' .. '30';
print "@$_" for sub{
    my $n=shift; 
    map{ [ splice @_, 0, $n ] } 0 .. @_ / $n 
}->( 5, @a );

01 02 03 04 05
06 07 08 09 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
26 27 28 29 30

print @a;
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2
+4 25 26 27 28 29 30

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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      <Tim "The Toolman" Taylor>

      Hrruuhhhh???

      </Tim "The Toolman" Taylor>

      While that's devilish and all that, how is it pathological? Heck, it doesn't even DWIM unless $n divides @_.

      The interesting bit, to me, is passing arguments to the anonymous sub.

      I guess I should have stated more explicitly that I'm looking for things that can't be grokked without deep diving into the docs or the source code. For instance, the continue/next behavior is documented, rather humorously, as "entertaining", without being so explicit as to spoil the fun.

      -QM
      --
      Quantum Mechanics: The dreams stuff is made of

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        The thing to note is that despite having "destructively" spliced the array into chunks, the array remains unchanged after the process.

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
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Re: Unobvious Pathological Code Snippets
by hv (Prior) on Mar 29, 2006 at 02:07 UTC

    Here's a couple:

      zen% perl -we '$|--; print $|; $|--; print $|; print --$|, --$|, $|-
+-, $|--, $/'
  100001
  zen%

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    Hugo

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