Use Firefox / Firebug to see what fields you are really posting. ASP adds many hidden fields. Go to Network > Post , right click and copy as POST args, try with LWP. Or copy as cURL request and try with cURL. If you don't get the results you expect, there's probably. JavaScript involved in posting the form and Mech won't be able to do the job. Then you'll need to use PhantomJS or Selenium. The latter can be controlled by Perl.

Hope this helps!