Doing it with pure CGI (ie, no Javascript), the entire page will need to be re-rendered to display the results. That may not be a big deal at all, however.

You might consider composing a CGI script that first renders the page if it discovers no input via the param() method. Next, after 'apply' has been clicked the same script will discover input on param(), and should perform the calculations, and output the page again, with the changes.

Using CGI.pm will simplify your life, of course.