As neniro points out, one of the benefits of using the OO interface for the CGI module is the fact that it doesn't cause namespace pollution. For large applications, that's an obvious advantage; for small applications less so.

Other benefits would include the potential to use multiple CGI objects, but why would you want to? Submit buttons can have values to distinguish them, which can be read as parameters if you give your buttons names.

As far as 3 goes, I think you're on your own and stuck with JavaScript. To achieve this, you need the client to perform the submit. As all of your Perl lives server-side, Perl doesn't have the ability to manipulate the client in this way.