Better solution (since it allows all keycards to be in use at the same time): 

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

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In reply to Re^2: Parallel::ForkManager and stack by ikegami
in thread Parallel::ForkManager and stack by marcpascual

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