It does not do anything with $thr.

Exactly. You are throwing it (them, one for each thread you create) away. So don't do that, save them.

my @threads;
for (0 ..3)) {
    my $thr = threads->create( 
        sub { worker(); } 
    );
    $thr->detach();
    push @threads, $thr;
}

... # then later
for my $thr ( @threads ) {
    print 'Thread ', $thr->tid, 
        $thr->is_running ? ' is running' : ' is not running';
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