Tom Christiansen's example code (à la perlthrtut) is a recursive, threaded implementation of finding and printing all prime numbers between 3 and 1000.

Below is a mildly adapted version of the script

#!/usr/bin/perl
# adapted from prime-pthread, courtesy of Tom Christiansen

use strict;
use warnings;
use threads;
use Thread::Queue;

sub check_prime {       
    my ($upstream,$cur_prime) = @_;     
    my $child;
    my $downstream = Thread::Queue->new;

    while (my $num = $upstream->dequeue) {          
        next unless ($num % $cur_prime);

        if ($child) {

            $downstream->enqueue($num);

        } else {

            $child = threads->create(\&check_prime, $downstream, $num);

            if ($child) {

                print "This is thread ",$child->tid,". Found prime: $num\n";

            } else {

                warn "Sorry. Ran out of threads.\n";
                last;
            }
        }
    }

    if ($child) {
        $downstream->enqueue(undef);
        $child->join;
    }
}

my $stream = Thread::Queue->new(3..shift,undef);
check_prime($stream,2);

When run on my machine (under ActiveState & Win32), the code was capable of spawning only 118 threads (last prime number found: 653) before terminating with a 'Sorry. Ran out of threads' warning.

In trying to figure out why I was limited to the number of threads I could create, I replaced the use threads; line with use threads (stack_size => 1);. The resultant code happily dealt with churning out 2000+ threads.

Can anyone explain this behavior?