Remember any such solution depends on the remote user's cooperation: that is, having the right version of JScript (which is not quite the same as JavaScript) activated. Remember that they could fake the name if they wanted. Never trust the client!

That said, another way of sending the information to your Perl script would be with a hidden image, whose URL would be something like "register_machine_name.cgi?name=whatever", where "whatever" is introduced by your client-side script.