test_forkmanager.pl

#!/usr/bin/env perl

use v5.18;
use Time::HiRes 'time';
use Parallel::ForkManager;

  my $pm = new Parallel::ForkManager(4); 
  my @links = 1..60;
  my $btime = time;
  foreach my $link (@links) {
    $pm->start and next; # do the fork
    say ((time-$btime),"\t",$link);
    $pm->finish; # do the exit in the child process
  }
  $pm->wait_all_children;

test_raw_fork.pl

#!/usr/bin/env perl

use v5.18;
use Time::HiRes 'time';


my @links = 1..60;
my $btime = time;
my $nfork = 0;
foreach my $link (@links) {
   my $pid = fork();
   die if not defined $pid;
   if ( $pid ){
     $nfork++;
     if ($nfork > 4){
       wait;
       $nfork--;
     }
   }else{
    sleep 2;
    say ((time-$btime),"\t",$link);
    exit;
   }
}