#!/usr/bin/perl

use strict;
use warnings;
use IO::Socket;

my $pid = fork();  die "couldn't fork" unless defined($pid);
if ($pid) {  # SERVER
    my $server = IO::Socket::INET->new(
        Proto     => 'tcp',
        LocalPort => 9999,
        Listen    => 1,
        ReuseAddr => 1,
    );
    die "couldn't setup server" unless $server;
    
    my $client = $server->accept();
    $client->autoflush(1);
    while (<$client>) {
        if    (/hello/) { print $client "nice to meet you\n" }
        elsif (/quit/)  { print $client "bye\n"; last }
        else            { print $client "??\n" }
    }
    close $client;

} else {     # CLIENT
    sleep 1; # wait for server to get ready
    
    my $sock1 = IO::Socket::INET->new( 
        Proto     => 'tcp',
        PeerAddr  => "localhost",  
        PeerPort  => 9999, 
    );
    die "couldn't connect" unless $sock1;
    
    open my $sock2, '+<&='.$sock1->fileno() or die $!;
    
    printf "sock1         = %s\n",   $sock1;
    printf "fileno(sock1) = %d\n",   $sock1->fileno();
    printf "autoflush     = %s\n\n", $sock1->autoflush() ? "ON":"OFF";
    
    printf "sock2         = %s\n",   $sock2;
    printf "fileno(sock2) = %d\n",   $sock2->fileno();
    printf "autoflush     = %s\n\n", $sock2->autoflush() ? "ON":"OFF";

    my $resp;
    print $sock2 "hello\n";
    $resp = <$sock2>;  print $resp;
    print $sock2 "quit\n";
    $resp = <$sock2>;  print $resp;
    exit;
}

##</code><code>##

$ ./751902.pl
sock1         = IO::Socket::INET=GLOB(0x7dc230)
fileno(sock1) = 4
autoflush     = ON

sock2         = GLOB(0x604410)
fileno(sock2) = 4
autoflush     = OFF

nice to meet you
bye