Insufficient data to be certain, but I'm betting you need to log in and have a cookie, or at least have a cookie jar to capture the cookie in. Check out
. If there's Javascript involved, you'll need a higher-tech solution: either to use a proxy (Live HTTP Headers is a good option) to see what the full HTTP interaction with the site is, and the duplicate that via LWP calls, or use
to do the download. (edit: whitespace)