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

my @teststrings = qw(ABCXXXXXX AXCXXXXXX AXXDXFXXX AXXXXXXHI XXXXXXGHI XXXXXXXXX AXXXXXXXI);

my $pattern = "ABCDEFGHI";
my $desired_matches = 3;

for (@teststrings) {
   my $result = $pattern ^ $_;
   my $matches = $result =~ tr/\0/ /;
   print "$_\n" if $matches == $desired_matches;
}