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;
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as been said, it's a race condition.

1. peter pops keycard and starts
2. james pops keycard and starts
3. john pops keycard and starts
4. luke pops keycards. It starts if one (or more) of peter, james or john has already finished. Otherwise it waits for one of them to quit. But what kind of keycard does it have?

Fixing the problems with forks (or threads) by randomly inserting calls to sleep is a TERRIBLE idea. Just don't ever do it!

It seems to me this problem could be solved by

my $pm = Parallel::ForkManager->new(2);
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or, more generally

my $pm = Parallel::ForkManager->new(@keycards - 1);
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All in all, that looks like a fairly questionable, error-prone feature of Parallel::ForkManager (which is a good module otherwise).