This looks like a winner for decoding. It handles the 'B' that indicates Base64 encoding. And it uses Encode's find_encoding to locate info on, say, 'Big5' charset.

On encoding author has an explicit apology that he only does encoding to UTF-8. But he does say he believes all mail clients should handle UTF-8 encoding these days, so maybe this shouldn't be a problem.

Hmmm, Encode says requires 5.7.3 or later. ggs can you use Perl 5.8 to run your program?