use threads;
use threads::shared;
use strict;
use warnings;

my $hellos_turn : shared = 1;
$| ++;

threads->create(sub {
    while (1) {
        sleep(1) until ($hellos_turn);
        print "Hello ";
        $hellos_turn = 0;
        threads->yield;
    }
});

threads->create(sub {
    while (1) {
        sleep(1) while ($hellos_turn);
        print "World!\n";
        $hellos_turn = 1;
        threads->yield;
    }
});

<STDIN>;