Perhaps you would find the book Web Client Programming with Perl, full text freely available from the O'Reilly Open Books Project, more useful than the WWW::Mechanize documentation.

Update: Although personally, I found the WWW::Mechanize::Cookbook helpful.

Update^2: Also, I really like using HTTP::Recorder to record a successful manual form submission. The output of HTTP::Recorder, just as with WWW::Mechanize::Shell, can be "dropped" right into your WWW::Mechanize scripts. (leira's article "Web Testing with HTTP::Recorder" contains an excellent example of how you might want to do this.)

Note that if you are dealing with javascript, you might need to use Win32::IE::Mechanize. If you run into problems, try Super Searching for "mech javascript" - there are many relevant nodes here.