I just posted a response to a similar question not to long ago. See: Re: Parallel Processing. Here's the sample code using Parallel::Runner

#!/usr/bin/perl

use strict;
use warnings;
use Parallel::Runner;

my $runner = Parallel::Runner->new(5);
$runner->run( sub { system("corelist CGI") });
$runner->run( sub { system("corelist CGI::Carp") });
$runner->run( sub { system("corelist Net::FTP") });
$runner->run( sub { system("corelist Net::SMTP") });
$runner->run( sub { system("corelist DBI") });
$runner->finish;
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