sub hamming {
  my ($x1, $x2) = @_;
  return -1 if length($x1) != length($x2);

  (my $xor = $x1 ^ $x2) =~ tr/\x0//cd;
}

print hamming(@$_), $/
   for [qw[ abcdef abccef ]],
       [qw[ abcdef abc ]],
       [qw[ abcdef abbbbf ]];
__END__
1
-1
3

##</code><code>##

# untested
my $target = "abcdef";
my $distance = 1;

my $len = length $target;
my $qr  = qr/ \b(\w{$target})\b (?(??{ hamming($target,$1) < $distance }) | (?!))  /x