I wrote a mini code for this problem which replicates my puzzle. 
#!/usr/bin/perl

use strict;
use Parallel::ForkManager;

my $pm = Parallel::ForkManager->new(10);

foreach my $j (1..100) {
    LINKS:
    for my $i (1..10) {
        $pm->start and next LINKS;
        my $num = count($i);
        print $num,"\t";
        $pm->finish;
    }
    print "\n";
    $pm->wait_all_children;
}

sub count {
    my $num = shift;
    return $num + 1;
}

[download]
I expected each row should generate ten numbers from 2 to 11. However, several rows will miss values. Even worse, several rows output 11 numbers instead of 10 numbers, and have replicate, for example, 10 appear twice. Why?

In reply to Re^3: missing values when using Parallel::ForkManager by Anonymous Monk
in thread missing values when using Parallel::ForkManager by plagent

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