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