For example, if you modify your code to return a random integer from 1 to 100 (in spawnThread), you could then make use of that value from the parent thread:

use warnings;
use strict;

use threads;
use threads::shared;
#use Time::HiRes;

#my $t0 = Time::HiRes::time;

my @array : shared;
for (1..50) {
    push @array, $_;
}

my $thr  = threads->create(\&spawnThread);
my $thr2 = threads->create(\&spawnThread);
my $thr3 = threads->create(\&spawnThread);
my $thr4 = threads->create(\&spawnThread);
my $thr5 = threads->create(\&spawnThread);

printf "Result = %d\n", $thr->join();
printf "Result = %d\n", $thr2->join();
printf "Result = %d\n", $thr3->join();
printf "Result = %d\n", $thr4->join();
printf "Result = %d\n", $thr5->join();

#my $t1 = Time::HiRes::time;
#my $elapsed = $t1 - $t0;
#print "\nElapsed Time: $elapsed\n";

sub spawnThread {
    for(1..10) {
        my $index = shift @array;
        print "$_ => $index\n";
    }
    return int rand(100);
}
[download]

Prints something like:

1 => 1
1 => 2
1 => 3
1 => 4
1 => 5

.
.
.

1 => 41
2 => 42
3 => 43
4 => 44
5 => 45
6 => 46
7 => 47
8 => 48
9 => 49
10 => 50
Result = 89
Result = 51
Result = 71
Result = 15
Result = 7
[download]

However in your original program, you weren't doing anything with the return value (in fact, you weren't even explicitly returning anything), so it would have been perfectly safe to call detach() on any or all of the threads, to avoid having to manage them further.

If your threads are running loops that don't have a definite completion point, you can used shared variables, to signal it to drop what it's doing, and return immediately.

One important difference between detaching and joining threads, (besides the return value which I've rarely found a need for), is that once you detach a thread, there is no architected way for the rest of your code to find out if that detched thread has terminated or not.

It means that if you detach threads, you will have to provide your own mechanism for detecting if they are still running, or risk terminating your program before they've stopped and receiving the "NN threads still running" warning.