Your solution will work, hawtin (I tried it for the famous "SEND + MORE = MONEY"), except that the operator precedence of $n.$d + $e.$s causes it to be grouped as $n.($d+$e).$s. A join('',...) might be more efficient anyway.

By the way, there's no need for a separate solution process for subtraction. You can just rearrange it into an addition problem.