$VAR1 = {
          'x' => 'cabd'
        };

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

my %hash = ( a => 11,
             b => 12,
             c => 13,
             d => 14,
           );

for my $key (keys %hash) {
    delete $hash{$_} for qw( a b c d );
    $hash{'x'} .= $key;
}
print Dumper \%hash;