Here's a link to some info for you, make what you will of it: http://perl.apache.org/docs/1.0/guide/debug.html#Handling_the__User_pressed_Stop_button__case

Combine that with, for example IPC::Run, and a pumping subprocess, so that you can kill the subprocess if you detect the socket close.

I would, however, caution you, in general, on going down this path, because proxy servers will break this. You may not have a proxy server in place now, but you may want to add one later. Also, the client may be behind a proxy, themselves, and you've got no control over that.