Are we talking empty string or undef here? They act differently, depending.

Normally they do, but we're talking about hash keys here. A hash key can only be a string, so any number, reference or undefined value is stringified. undef happens to stringify to the emtpy string, so $foo{+undef} and $foo{''} are the same thing.

2;0 juerd@ouranos:~$ perl -le'$foo{+undef} = "Hello, world!"; print $f
+oo{""}'
Hello, world!
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- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

Which is shown nicely by:

#!/usr/bin/perl

use Data::Dumper;

$hash{+undef} = "pleasure";
$hash{''} = "pain";

print Dumper(\%hash);
[download]

Outputs:

$VAR1 = {
          '' => 'pain',
        };
[download]

--
Steve Marvell

Update: I tried to cancel the submission and ended up in another double submission fiasco. I realied my error, hence the latter post.

--
Steve Marvell

sub bar () { 'quux' }

$foo{undef} = 'bareword undef is treated as string';
$foo{+undef} = 'function undef is treated as function because +
                is not valid in a bareword string';
$foo{bar} = 'key bar is used';
$foo{+bar} = 'key quux is used';
$foo{bar()} = 'key quux is used again (the old value is overwritten)';
[download]

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

If the stuff inside the curlies looks like a word it's automaticaly quoted.

Thus $hash{undef} and $hash{'undef'} are equivalent.Try :

$ perl -MO=Deparse -e "print $hash{undef}"
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If you want to use an undef as a hash key you have to use either $hash{+undef} or something like my $NULL = undef; $hash{$NULL}

Update: Erm ... hash keys are strings. Always. So $hash{+undef} us equivalent to $hash{''}. Oh well.

  Jenda