Whaouh! XML::Twig 2.02 is pretty old! You should definitely upgrade, provided you dont (ab)use tricks like including mark-up in the text of elements (see the Changes file): provided you use the keep_encoding option, the new version will get the attributes in the original encoding, not in UTF-8.

As far as guessing the encoding of a string, you can try Encode::Guess, but it might not work with 5.6.1 (it is part of 5.8.0 core) and, as stated by the author Use this module with care.