The first thing that I can think of is trying to open a socket to that host:port.

use strict;
use IO::Socket;

my $host = "localhost";
my $sock;
print "Open ports:\n";
for(my $port=1; $port < 1024; $port++) {
    $sock = new IO::Socket::INET(PeerAddr => $host, PeerPort => $port,
+ Proto => 'tcp');
    if ($sock) {
        print "$port\n";
        close $sock;    
    }
}
[download]

Hope this helps.
marcos