I have no straight answer to your question. From my experience working with UTF-8 i can say that i had no trouble with browser requests: when i sent page in UTF-8 i got it posts back in UTF-8. But i had lots of troubles with unicode dataflow in Perl itself. Still i suggest certainly use UTF-8 (not old ISO-8859-x). About using UTF-8 i wrote a node 731943, there you can see my problems and some good comments for workarounds. I hope it will give you some additional bits to thought.
in reply to UTF-8 or iso-8859-1 input to CGI.pm