for my $c (@car_links) {
    threads->create(sub {
        threads->detach;

        # Reserve for 30 min. total.
        for (1..6) {
            reserve_car($c, $username, $password);
            sleep 300;
        }
    });
    
    push @known_cars, $c;
    write_url_to_known_cars($c, $fh);
}