Hi, I'm trying to pop elements from an array in parallel until it consumes everything, then have the forks return it back for the next consumers. I can't figure out why I keep on getting the uninitialized value error (below). I thought adding a sleep before the start() would help (I am probably misunderstanding the problem as forks being too fast) but it only helped to a certain extent (compared to not having a sleep at all). 
#!/usr/bin/perl

use strict;
use warnings;

use Parallel::ForkManager;

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

my $pm = Parallel::ForkManager->new(3);
$pm->run_on_finish(
    sub {
        my $key = $_[5]->{key};
        unshift(@keycards, $key);
    }
);

foreach my $user (@users) {
    sleep(int(rand(3)));
    my $key = pop(@keycards);
    $pm->start and next;
    print "$user took the '$key' key\n";
    $pm->finish(0, { key => $key });
}
$pm->wait_all_children;

[download]
[user@localhost ~/bin]$ perl a.pl
peter took the 'C' key
james took the 'B' key
john took the 'A' key
luke took the 'C' key
andrew took the 'B' key
judas took the 'A' key
[user@localhost ~/bin]$ perl a.pl
peter took the 'C' key
james took the 'B' key
john took the 'A' key
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 23.
luke took the '' key
andrew took the 'C' key
judas took the 'B' key

[download]

In reply to Parallel::ForkManager and stack by marcpascual

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