I am not aware of any cross-platform way to set thread priority. However you can limit the number of threads you create by simply checking how many threads you get from threads->list and waiting to spawn a new thread until scalar(threads->list) < $max_threads.

Something like:

use threads;

my $max_threads = 5;
for( 1..20 ) {
  sleep(1) until scalar(threads->list) < $max_threads;
  threads->create( \&do_work );
}

# Now wait for the remaining threads to finish:
$_->join foreach threads->list;
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