Finding hidden backdoors

I saw some people talking about rootkits that hidden process/ports. One think that i always do to see what ports are open is to run this perl script:

use IO::Socket;
for($i=0;$i<=65555;$i++)

        {
        $server[$i] = IO::Socket::INET->new(
        Proto => 'tcp',
        LocalPort => $i,
        Listen => SOMAXCONN,
        Reuse => 1) or print "Port $i Open \n" unless $server[$i];
        close ($server[$i]);
        }

This is good because if "netstat" or "lsof" or "fuser" or any other program is trojaned , or if it has any firewall and nmap is not finding all the open ports, this script will show ... The other benefit is that you cant hidden from it using any LKM code... What do you thing ?

thanks

Daniel B. Cid