Correction. It didn't work but for a different reason. I had a type from the first try. When I tried you method with the uri post it still prints out the main page when I do a print $res->content.
Be sure you've had a look at the source code for the login web page and have the field names for the login fields set correctly. Alternatively, go with the suggestions to use WWW:Mechanize -- I haven't used it, but I hear it simplifies matters greatly.