IIRC this is only possible for Perl running under modperl. perrin will correct me if that's wrong. :) I Googled around a bit on this theme but didn't find what I remember reading 5 years ago.

I doubt that is what you want though. For the general case, if it's really important to clear the file, I'd probably just add a method to your CGI which sends a follow-up "DELETE/POST" when you click "cancel" to remove the file that was just uploaded with the specified identifier/timestamp.