The reason you can't figure it out is because it's a JavaScript question, not a Perl question.

JavaScript is a client-side language, which can control the browser. Perl is the server-side language which, when told to do so by the browser, will do the math and send back the result.

How does your interface work, by the way? Can any of your menus make the form submit? If I want to do two plus two equals four, do I have to select two, have the form submit, select plus, have the form submit, select two again and have the form submit a third time? That's going to drive me crazy...