Regardless of whether or not the page uses Javascript it still has to communicate with the server via HTTP. All you need to do is find out if the Javascript is affecting what is sent to the server or not. You might also have to step outside of WWW::Mechanize and use HTTP::Request your self. It's really not that bad, and you can even use the two together.

  #build out your request
  my $request = HTTP::Request::Common::POST->new(....);
  $mech->request($request);
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And then you can continue using Mech.

If you are having a hard time figuring out what HTTP the browser is sending to the server, check out something like the LiveHTTPHeaders plugin for Mozilla, or ethereal.