My boilerplate advice is this:

I would try using a module such as HTTP::Recorder or WWW::Mechanize::Shell to record a successful manual form submission. The output of HTTP::Recorder, for instance, can be "dropped" right into your WWW::Mechanize scripts.

I am also on Win32; I have had no problems using HTTP::Proxy, so that may be a possibility for you.

Another important tool for finding out what is really happening behind the scenes between server and browser is a protocol analyzer such as Ethereal.

Don't forget that Super Search is your friend here on PM. Many questions such as yours have been asked recently...

HTH,