Thanks mattk
In fact I am using fork() on Windows (my bad). However, my problem reduces to something like this:
My server:

use IO::Socket;
use IO::Select;

my $socket= new IO::Socket::INET (
        LocalHost => 'localhost',
        LocalPort => 9000,
        Proto => 'tcp',
        Listen => SOMAXCONN,
        Reuse => 1);
if (not defined $socket) {
        die "no server";
}
my $sel= IO::Select->new($socket);

print "Waiting for connections on port 9000\n";
    
my @ready;
my $client;
my $message;
my $new;
while (@ready= $sel->can_read) {
    foreach $client (@ready) { #client speaking
        if ($client == $socket) { #new client -> add client
            print "new client\n";
            my $add= $client->accept;
            $sel->add($add);
            print "added\n";
        } else { #old client 
            print "old client \n";
            $client->recv($message,256);
            print $client."\n";
            $client->send($client);
            $client->recv($new, 256);
            print $new."\n";
            $new->send("ok");
        }
    }
}
[download]

My client:

my $sock;
my $message;


my $socket= new IO::Socket::INET (
        PeerHost => 'localhost',
        PeerPort => 9000,
        Proto => 'tcp');
if (not defined $socket) {
        die "no server";
}


$socket->send("something");
$socket->recv($sock, 256);
$socket->send($sock);
$socket->recv($message, 256);
[download]

And this gives me something like:
Waiting for connections on port 9000 new client added old client IO::Socket::INET=GLOB(0x1b19628) IO::Socket::INET=GLOB(0x1b19628) Can't locate object method "send" via package "IO::Socket::INET=GLOB(0x1b19628)" (perhaps you forgot to load "IO:Socket::INET=GLOB(0x1b19628)"?) at server.pl line 39