多线程 – 强制perl程序使用线程来使用所有CPU容量

我的代码：

use strict;
use warnings;
use threads;
use threads::shared;
print "Starting main programn";

my $t1 = threads->create(&;sub1,1);
my $t2 = threads->create(&;sub1,2);
my $t3 = threads->create(&;sub1,3);
my $t4 = threads->create(&;sub1,4);

$t1->join();
$t2->join();
$t3->join();
$t4->join();
print "End of main programn";
sub sub1 {
my $num = 20;

print "started thread $numn";
sleep $num;
print "done with thread $numn";
return $num;
}

但运行后CPU使用率约为10％.

我如何使用100％的CPU？