WWW::Mechanize doesn't handle Javascript. So you'll either have to find out what the Javascript does, and replicate that in your Perl code. Or you have to instrument a browser, by using Win32::IE::Mechanize or WWW::Mechanize::FireFox.

While I'm the author of WWW::Mechanize::FireFox, and it works very well for me, I'll note that the Mechanize API is very incomplete. But I use it to automate Javascript-heavy web applications.