Check :
- If you get a 30x HTTP Response from the server, for LWP doesn't handle POST as the IIS server expect when there's a redirection (look at Redirect_ok)...
- Fake the UserAgent to an usual one 'Mozilla..'
- Fake a referer (setting it to the page where the form is...)
In last resort use SocketWorkbench to find what's different with a posting via your browser...