Well, since an internal server error happens on the server, it usually means the script the page was submitted to did not receive all the information it was expecting (and it has poor error handling). Often this is a cookie or state variable problem, but in this case I think it is a javascript issue.

The login button on the saxobank page runs some javascript, but LWP does not interpret javascript, so the javascript doesn't do it's work. I would guess this javascript is vital to the site, and the cause of your error.

The javascript looks like it is just changing the form action location, so you should be able to fake it out with some work on the perl side.