I think what you are talking about is filling in the contents of the form without refreshing the entire page. To do this, you need to do some reading up on AJAX, specifically the XMLHttpRequest object. A good place to start is at the w3schools AJAX Intro.

The oversimplified answer is that in the onChange event of the input field, you set up the request, which will call your Perl script on the server. When the results come back, you parse the results and fill out the rest of the form. It's not at all difficult to do something this simplified, but for anything more complicated you'd be better off using a javascript library like Prototype or dojo.

Edit: Added AJAX tutorial link