sub hamming_distance_string{
        #String length is assumed to be equal
        my ($a,$b) = @_;
        my $len = length ($a);
        my $num_match=0;

        for (my $i=0; $i<$len; $i++) {
             ++$num_match if substr($a, $i, 1) eq substr($b, $i, 1);
        }

        return $num_match;
}