Perl is (nearly always) server-side. The web browser is client-side. The two communicate via HTTP requests, not window events. Either the user must send information from the client to the server via following a link or hitting a form submission button, or some sort of client-side scripting (JavaScript) will force the client to send another request.

Unless you have some sort of push session in the main window and use a Perl script on the server to receive a form submission from the popup window, you're probably not going to find a way to do it in Perl.

JavaScript and the DOM are the quickest way to get this done -- at least, to my knowledge.