Assuming you are using a module such as WWW::Mechanize (and I really can't help you much without knowing what you are using - that's where a snippet of your code would be really helpful):

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

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...

HTH,