If I understand correctly, you want to change the settings of controls on the form after it's been rendered. Neither CGI nor anything else on the server can do that, you'll have to use something on the client to handle it, probably javascript.

(There is the CGI::Ajax module to allow you to easily write server-side code to control it, but it works by inserting javascript in the page telling it to call the server when the changes are triggered. IMO, going to AJAX for simple, purely-display logic like this is overkill, though.)