It isn't, at least not directly. (Unless you are using a client side PerlScript plugin (which I've never seen available for a browser other then MSIE and never seen used in the wild).)
There is an indirect approach though.
- When it fires make an HTTP request (e.g. with XMLHttpRequest or setting the location property).
- Use the information in the request in your server side program to run the routine you want.
A JS library such as YUI will do most of the heavy lifting for you. Make sure you use progressive enhancement to ensure your system is robust.