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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