G'day amitsq,

I suspect your problems are related to the additional $pm->finish and next statements.

Your instantiation of the Parallel::ForkManager object may also be an issue. What you have is different from every example shown in the documentation; also see "perlobj: Invoking Class Methods" (including its "Indirect Object Syntax" subsection).

Using exactly the same message to identify your different cases is pointless: use meaningful messages.

I rewrote your code like this (pm_1199760_parallel_forkmgr_link_checker.pl):

#!/usr/bin/env perl -l

use strict;
use warnings;

use LWP::Simple;
use Parallel::ForkManager;

my @urls = qw{
    http://us.a1.yimg.com/us.yimg.com/i/ww/m5v9.gif
    http://hooboy.no-such-host.int/
    http://www.yahoo.com
    http://www.ora.com/ask_tim/graphics/asktim_header_main.gif
    http://www.guardian.co.uk/
    http://www.pixunlimited.co.uk/siteheaders/Guardian.gif
};

my $max_processes = $ARGV[0] || 8;

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

print 'Start link checking.';

for my $url (@urls) {
    $pm->start and next;

    my ($type, undef, $mod) = head($url);

    if (defined $type) {
        print "PASS: $url";
        print '  MOD: ', $mod || 0;
    }
    else {
        print "FAIL: $url";
    }

    $pm->finish;
}

$pm->wait_all_children;

print 'Link checking completed.';
[download]

Here's the output with no argument:

$ pm_1199760_parallel_forkmgr_link_checker.pl
Start link checking.
FAIL: http://hooboy.no-such-host.int/
PASS: http://us.a1.yimg.com/us.yimg.com/i/ww/m5v9.gif
  MOD: 1354074654
PASS: http://www.guardian.co.uk/
  MOD: 0
FAIL: http://www.ora.com/ask_tim/graphics/asktim_header_main.gif
PASS: http://www.yahoo.com
  MOD: 0
FAIL: http://www.pixunlimited.co.uk/siteheaders/Guardian.gif
Link checking completed.
[download]

I then ran it with arguments of 8, 5, 3, 2 and 1. Lke the first run (with no argument), all ran to completion: no crashes! With the exception of 1, they produced the same results (although the order of output varied).

Running with an argument of 1 rather defeats the purpose of running processes in parallel; however, as you said you'd used it, I tried it also. Here's what I got:

$ pm_1199760_parallel_forkmgr_link_checker.pl 1
Start link checking.
PASS: http://us.a1.yimg.com/us.yimg.com/i/ww/m5v9.gif
  MOD: 1354074654
FAIL: http://hooboy.no-such-host.int/
PASS: http://www.yahoo.com
  MOD: 0
FAIL: http://www.ora.com/ask_tim/graphics/asktim_header_main.gif
FAIL: http://www.guardian.co.uk/
FAIL: http://www.pixunlimited.co.uk/siteheaders/Guardian.gif
Link checking completed.
[download]

I ran that a second time: the result was the same. I'll leave you to investigate that further.