use warnings;
use strict;

my $source="ATTCCGGG";
print "source: $source\n";
for my $str ( "ATTGCGGG", "ATACCGGC", "ACTGCGGT" , "ATCGGGGG" ) {
    print "string: $str\n";
    my @res = unpack "c*", $source ^ $str;
    for my $i (grep $res[$_], 0 .. $#res) {
        print "$i: ", substr($source, $i, 1), "->",
            substr($str, $i, 1), "\n";
    }
}