Thank you for your continued interest in my problem.The initial URL takes you to a login page where you are prompted to enter the userID and password. It is not a pop-up window.
Don't know if that makes a difference regarding using LWP. However this morning, I was trying to figure out what was going on by looking at the response headers and it looks like the session cookie that is sent back after the initial GET request is getting sent back in the header of the POST request. However, the server then sends back a second session cookie, and therein lies the problem, as this probably means that the server does not get the userID, password and session ID that is being sent to it with the POST.
I have to temporarily stop working on this but will get back and try installing WWW::Mechanize and see. Will keep you posted.