I've also dealt with some sites where authtentication is just a form. For example, this was enough to get me into one:

use HTTP::Request::Common;

my ($user, $password);  # Set to whatever ...
my $request = POST("http://www.whatever", 
                   Content => [ UserID => $user,
                                Password => $password,
                              ]);

# Feed the request to LWP::UserAgent, etc.
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