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my @keycards= qw(A B C);
my @users = qw(peter james john luke andrew judas);
...
    sleep(int(rand(3)));
    my $key = pop(@keycards);
    $pm->start and next;

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

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