use strict;
use warnings;

my %strings = (
    'undef' => undef,
    'empty' => '',
    'string' => '0'
);

my @pairs = (
    [ 'undef' , 'undef'  ],
    [ 'undef' , 'empty'  ],
    [ 'undef' , 'string' ],
    [ 'empty' , 'empty'  ],
    [ 'empty' , 'string' ],
    [ 'string', 'string' ],
);

for ( @pairs ) {
    my ($p,$q) = @strings{@$_};
    my $cmp = ( (defined($p) ? '1' . $p  : 0) 
                eq
                (defined($q) ? '1' . $q  : 0)    
              ) ? 'true' : 'false';
    print join(" eq ", @$_), " = $cmp\n";
}