This is really a JavaScript question. You can indeed write the dropdowns in Perl and might want to depending on what you're doing but to dynamically update (reveal, hide, switch lists) the dropdowns you have to use JS.

I highly recommend jQuery for this. It's ideal for this sort of DOM manipulation. You could write all the fields to the page in your CGI with Perl and hide them with CSS, then use JS to reveal/switch them as you need.