By embedding the username and password in the URL you're using a method of passing query info that is typical of the GET method. I'm not shocked at all that ASP doesn't like you mixing the GET and POST method in a single request. That's why it's complaining- your authentication information is being sent using the GET method, embedded in the query string.

Does the site use cookies to track user authentication? If so, you need to set up a cookie jar using HTTP::Cookies to save the cookie, which will then be passed in all subsequent requests of the same useragent object.

Ugh. That might be confusing. Just do a google search for "lwpcook" (or "man lwpcook") to find the excellent LWP cookbook. It will give you a much more elegant description of how to do this.

-Any sufficiently advanced technology is
indistinguishable from doubletalk.