There are two approaches to your problem, one completely unrelated to Perl and one marginally related to Perl:

If you want to send a database query after some item has been selected without reloading the whole page, you will have to look at how Orkut (for example) does this with the XMLHTTPRequest object. This is a possibly convenient way of doing things, but it locks out all people who have disabled JavaScript and all people not using a recent browser, as there is no implicit graceful fallback.

If you can instead load the whole data in one go and only later update the contents of the popup list based on the selection in another list, then creating that page is easily done with Perl and one of the templating modules. The problem remains how to change the contents of the popup list. Google gives this link, possibly you will find better links by using Google yourself.