For fun, here's an example of using undef as a hash key.  It validates what Joost and Eimi Metamorphoumai have said, that as a key, an undef is equivalent to the empty string "": 
#!/usr/bin/perl -w

use strict;
use warnings;
use Data::Dumper;

my $x;    # $x is undef
my $y;    # $y is undef too

my %hash;
$hash{$x} = 123;

# This will throw warnings, but both $hash{$x} and $hash{$y} are 123
printf "\$hash{\$x} = %s; \$hash{\$y} = %s\n", $hash{$x}, $hash{$y};

print "Keys of \$hash => %s\n", Dumper([keys %hash]);

__END__

Output:

Use of uninitialized value in hash element at x line 11.
Use of uninitialized value in hash element at x line 11.
Use of uninitialized value in hash element at x line 14.
Use of uninitialized value in hash element at x line 14.
$hash{$x} = 123; $hash{$y} = 123
Keys of $hash => $VAR1 = [
          ''
        ];

[download]
s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/

In reply to Re: undef as a key in a hash. by liverpole
in thread undef as a key in a hash. by fenLisesi

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