there exists a technique called pseudo-server-push, which means that one script is running all the time sending output to the browser in an unbuffered way. The problem with this technique is that if the stream breaks (because of proxies), no data is transferred any more.

Chats who use this technique is Ralfchat (only in combination with the database; but the code is not scambled) or GtChat (but the code is scrambled)

If I'd write a webchat, I'd think about Flash, because there you can easily set up a Socket-Client that can communicate easily with a perl-socketserver...