use threads;
use threads::shared;

use constant NUM_WORKERS => ...;

sub process {
   my ($node) = @_;
   ...
}

{
   my $tree: shared = ...;

   my $q = Thread::Queue->new(NUM_WORKERS);

   for (1..NUM_WORKERS) {
      async {
         while (my $node = $q->dequeue()) {
            process($node);
         }
      };
   }

   my @todo = $tree;
   while (@todo) {
      my $node = pop(@todo);
      $q->enqueue($node);
      push @todo, $node->children();
   }

   $q->end();
   $_->join() for threads->list();
}