Basically, you can only use perl functions with perl... you can't use perl functions in html.
To make this work, I would first check the parameter and if it exists, then print out your information, otherwise just display your form. Alternatively you can just launch a new cgi page, it all depends on what you want to do.
Just remember, your perl code generates html. A drop down list is part of that html. You can take the parameter of that dropdown and parse it in another form or the same form but you have to resubmit the form. (which you can do on selecting an item from a dropdown)