sub really_big_form {
    my $q = shift;

    my $sid = $q->param('session') or goto sub {
        print_error('Get a session, loser');
    };

    $session_vars{$sid}{new_style} or goto sub {
        respawn_session($sid);
        print_error('Old-style sessions are not cool, you are
        upgraded, go check your settings right now');
    };
    # ...
}