server.pl:
use IO::Socket::INET;

$server = new IO::Socket::INET(Timeout => 20, Proto => "tcp", LocalPort => 3000, Listen => 5)
          || die "failed to establish socket\n";
$client = $server->accept;
while (1) {
    sleep(2);
    if ($client->connected) {
        print "still connected\n";
    } else {
        print "disconnected\n";
    }
}

client.pl:
use IO::Socket::INET;

$client = new IO::Socket::INET(Timeout => 20, Proto => "tcp", PeerPort => 3000, PeerAddr => "localhost")
          || die "connection failed";
while (1) {
    sleep(2);
    if ($client->connected) {
        print "still connected\n";
    } else {
        print "disconnected\n";
    }
}