#!/usr/bin/perl

use strict;
use warnings;

use Parallel::ForkManager;

my @keycards= qw(A B C);
my @users = qw(peter james john luke andrew judas);

my $pm = Parallel::ForkManager->new(3);
$pm->run_on_finish(
    sub {
        my $key = $_[5]->{key};
        unshift(@keycards, $key);
    }
);

foreach my $user (@users) {
    sleep(int(rand(3)));
    my $key = pop(@keycards);
    $pm->start and next;
    print "$user took the '$key' key\n";
    $pm->finish(0, { key => $key });
}
$pm->wait_all_children;