use threads;

my $thr = threads->create(\&sub1);   # Spawn the thread

$thr->detach();   # Now we officially don't care any more

sleep(15);        # Let thread run for awhile

sub sub1 {
    $a = 0;
    while (1) {
        $a++;
        print("\$a is $a\n");
        sleep(1);
    }
}