Thanks for the reply. At first I thought that would work, too. Specifically, I tried:

$mech_obj->submit_form( form_number => 2,
                        fields => { 'username' => $username,
                                    'password' => $password }
                       );
[download]

But I got the same result as when I used the submit method directly: the search box (form 1) was submitted instead (despite form 2 being selected). Thanks for trying, but I'm still looking for a solution.