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for my $c (@car_links) {
    my $pid = fork();
...
    push @known_cars, $c;
    write_url_to_known_cars($c, $fh);
}

for my $c (@car_links) {
    threads->create(sub {
...
    push @known_cars, $c;
    write_url_to_known_cars($c, $fh);
}