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

foreach my $user (@users) {
    sleep(int(rand(3)));
    my $key = pop(@keycards);
    $pm->start and next;

##</code><code>##

my $pm = Parallel::ForkManager->new(2);

##</code><code>##

my $pm = Parallel::ForkManager->new(@keycards - 1);