Neither MIME::Lite (basically deprecated now) nor Email::MIME seem to fit this desire. They issue a "wide char in print" warning on a :raw filehandle for my test with embedded katakana, which means they returned a non-encoded Unicode string, as I understand it

Code please, not words :)

On second thought, you know who MIME::Lites big daddy is? http://search.cpan.org/~dskoll/MIME-tools-5.502/MANIFEST

I would give that a shot as the test suite contains http://cpansearch.perl.org/src/DSKOLL/MIME-tools-5.502/testmsgs/attachment-filename-encoding-UTF8.msg