http://www.perlmonks.org?node_id=747505


in reply to UTF-8 or iso-8859-1 input to CGI.pm

If you don't trust the browser, I'd try to find a named entity that's represented as a different value in each of the two encodings, and set it as a value in a hidden field.

If there aren't any such characters, you could at least use a character that's in unicode, but not iso-latin, and see what comes back from the browser.