If you're going to use WWW::Mechanize, write

$agent->get($url);
if( $agent->success ){
    $agent->form_number(1);
    $agent->form_with_fields(qw' username password ');

    $agent->field("username", "---------");
    $agent->field("password", "---------");
    
    $agent->submit;

    if( $agent->success ){
        die "I logged in, now I die";
    }
}
[download]

remember, WWW::Mechanize is Handy web browsing in a Perl object :D