Great mind, great minds ... ;)

Here's the test program I wrote:

#!/usr/bin/perl -l

use strict;
use warnings;
use Math::Combinatorics;

my %v = (
    undef => undef,
    empty => '',
    zero  => 0,
    name  => 'Ovid',
);

my $combine = Math::Combinatorics->new(
    count => 2,
    data  => [ keys %v ],
);

while ( my ( $x, $y ) = $combine->next_combination ) {
    compare( $x, $y );
}

foreach my $x ( keys %v ) {
    compare( $x, $x );
}

sub compare {
    my ( $x, $y ) = @_;
    my ($X, $Y) = ($v{$x}, $v{$y});
    print "\nComparing '$x' and '$y'";
    no warnings 'uninitialized';
    print +( ( defined $X == defined $Y ) && $X eq $Y )
      ? "yes"
      : "no";
}
[download]