I'd use XOR logic. 
# $m      = 5
# $a      = "ABCDEFGHIJKL"
# $b      = "ACHDEFIHAJLK"
# $diff   = 0
# $xor_ed = "\000\001\013\000\000\000\016\000\010\000\007\007"

sub mdiff {
  my ($m,$a,$b) = @_;
  my ($la,$lb) = (length $a, length $b);
  my $xor_ed;
  my $diff = abs($lb - $la);

  return 1 if $diff >= $m;
  return 1 if ($diff + ($xor_ed = $a ^ $b) =~ tr/\0//c) >= $m;
}

[download]
Bitwise XOR in strings is a beautiful thing. The number of NULs in the XOR-ed string (\0) will be the number of same characters. The opposite is the number of different characters, so I tr/\0//c.

In reply to Re: diff (different enough) by japhy
in thread diff (different enough) by jettero

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