my $manager = Parallel::ForkManager->new(4);
    for(@identifier){ #List of URLs to be crawled
        $manager->start and next;

        $mech->get($_);
        die $mech->response->status_line unless $mech->success;

        my $html = $mech->content;

        ##some processing of HTML to extract the location of PDF file## 

        mirror($url,"/home/username/data/$file_name.pdf");

        $manager->finish;
        sleep(2)
    }
    $manager->wait_all_children;