Just as an aside ... looking at your code, why not simply write

my @threads;
for my $i (0..19) {
  print "THREAD SPAWNED.\n";
  $threads[$i] = threads->new(\&mainsub, $i);
  $threads[$i]->detach;
  sleep 1;
}
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instead of this

print "THREAD SPAWNED.\n";
$thr1 = threads->new(\&mainsub,0);
$thr1->detach;
sleep 1;

print "THREAD SPAWNED.\n";
$thr2 = threads->new(\&mainsub,1);
$thr2->detach;
sleep 1;

# ...
[download]

'red flag' code repetition.