Technically, you cannot set a thread limit with semaphores this way. It does work in practice, but there's no real guarantee that a thread, or all threads, aren't preempted right after the $sem->up().

Second point, do not detach a thread if you need to wait upon it. Grab its end result (return value) with a join(), instead.

You may not want to create too many threads (especially with fine-grained jobs). An alternative that might be suitable is to arrange for a conveyor belt with N workers. Use Thread::Queue for that. Here's a simple, if overly recursive, example: 

use threads;
use Thread::Queue;

my $WQ = Thread::Queue->new('unit001' .. 'unit123');
sub qrun {
    map { $_ ? (worker($_), qrun()) : () } $WQ->dequeue_nb;
}
sub worker {
    warn "Work $_";
    select(undef, undef, undef, rand 3);  # sleep some
    return "$_!";
}

print for
map { $_->join } map { threads->new(\&qrun) } 1 .. 15;

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In reply to Re: Threading and join/termination question by oiskuu
in thread Threading and join/termination question by cormanaz

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