#!/usr/bin/perl -w
use strict;

my $s1 = 'AAAAA';
my $s2 = 'ATCAA';
my $s3 = 'AAAAA';

hd($s1,s2) # will give value 2
hd($s1,s3) # will give value 0

sub hd
{
     #String length is assumed to be equal
     my ($k,$l) = @_;
     my $len = length ($k);
     my $num_mismatch = 0;

     for (my $i=0; $i<$len; $i++)
     {
      ++$num_mismatch if substr($k, $i, 1) ne substr($l, $i, 1);
     }

     return $num_mismatch;
}