Update: Modified script. Completes in 1 second with Cygwin Perl v5.22.4 and less than 1 second on Unix.

Update: Modified list of modules to pre-load. On the Windows platform, run with Perl v5.26 or later for best results.

Running parallel is problematic and not fun on the Windows platform. To increase the chance for failure, I quadrupled the input size.

# Strawberry Perl on Windows 7 VM
# LWP::Simple included with Perl
# Parallel::ForkManager v1.19

# Testing involves running multiple times.
# Failing indicates the script crashed one or more times.

perl-5.10.1.2 - LWP::Simple v5.827 : pass, slow (> 14 seconds)
perl-5.12.3.0 - LWP::Simple v5.835 : pass, slow (> 14 seconds)

perl-5.14.4.1 - LWP::Simple v6.00  : fail
perl-5.16.3.1 - LWP::Simple v6.00  : fail
perl-5.18.4.1 - LWP::Simple v6.00  : fail
perl-5.20.3.3 - LWP::Simple v6.15  : fail
perl-5.22.3.1 - LWP::Simple v6.15  : fail
perl-5.24.2.1 - LWP::Simple v6.26  : fail

perl-5.26.0.2 - LWP::Simple v6.26  : pass, fast (~ 3 seconds)
[download]

A solution is pre-loading essential modules (required at runtime) by LWP::Simple before running parallel.

use strict;
use warnings;

use LWP::Simple;

# Pre-load essential modules for extra stability.

if ( $INC{'LWP/UserAgent.pm'} && !$INC{'Net/HTTP.pm'} ) {
   require IO::Handle;
   require Net::HTTP;
   require Net::HTTPS;
}

my @urls = (
   'http://hooboy.no-such-host.int/',
   'http://us.a1.yimg.com/us.yimg.com/i/ww/m5v9.gif',
   'http://www.guardian.co.uk/',
   'http://www.ora.com/ask_tim/graphics/asktim_header_main.gif',
   'http://www.pixunlimited.co.uk/siteheaders/Guardian.gif',
   'http://www.yahoo.com',
) x 4;

use Parallel::ForkManager;

my $pm = new Parallel::ForkManager(8);

if ( $^O ne 'MSWin32' ) {
   $pm->set_waitpid_blocking_sleep(0);
}

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

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

   # if (!defined $type) {
   #    ...
   # }
   # elsif ($mod) {
   #    ...
   # }
   # else {
   #    ...
   # }

   print "$url is done\n";

   $pm->finish;
}

$pm->wait_all_children;
[download]

Results.

# Strawberry Perl on Windows 7 VM
# LWP::Simple included with Perl
# Parallel::ForkManager v1.19

# Testing involves running multiple times.

perl-5.10.1.2 - LWP::Simple v5.827 : pass, > 14 seconds
perl-5.12.3.0 - LWP::Simple v5.835 : pass, > 14 seconds
perl-5.14.4.1 - LWP::Simple v6.00  : pass, >  7 seconds
perl-5.16.3.1 - LWP::Simple v6.00  : pass, >  7 seconds
perl-5.18.4.1 - LWP::Simple v6.00  : pass, >  7 seconds
perl-5.20.3.3 - LWP::Simple v6.15  : pass, >  6 seconds
perl-5.22.3.1 - LWP::Simple v6.15  : pass, >  6 seconds
perl-5.24.2.1 - LWP::Simple v6.26  : pass, >  6 seconds
perl-5.26.0.2 - LWP::Simple v6.26  : pass, ~  3 seconds

perl v5.22.4 on Cygwin - LWP::Simple v6.27 : pass, 1 second  ;-)
[download]

Perl 5.26 provides the best performance, completing in 3 seconds.

Regards, Mario