If you're thinking along the lines of a webpage-based chat system (like the CB here on Perl Monks), then I'd say go for the Java. Having to refresh an entire web page every time you want to update the conversation is quite inefficient. Java applets can offer a true real-time chat.

If you're not limited to doing this within a browser, than you have a stronger case for Perl. Or just document the protocol well and implement in both languages.