#!/usr/bin/perl

use IO::Socket::INET;

use strict;
use warnings;

if (my $pid = fork()) {
    my $listen = IO::Socket::INET->new(
        LocalAddr => 'localhost',
        LocalPort => 9000,
        Proto    => 'tcp',
        Listen   => 1,
    ) or die "Cannot create listening socket: $!\n";

    my $client = $listen->accept()
        or die "Cannot get a connection: $!\n";

    $client->autoflush(1);

    while (defined(my $line = <$client>)) {
        print "server < $line";
        sleep 1;
        pong($client);
    }
}
else {
    print "waiting for parent to setup...\n";
    sleep 1;

    my $sock = IO::Socket::INET->new(
        PeerAddr => 'localhost',
        PeerPort => 9000,
        Proto    => 'tcp',
        Type     => SOCK_STREAM
    ) or die "Cannot create connecting socket: $!\n";

    $sock->autoflush(1);

    ping($sock);
    while (defined(my $line = <$sock>)) {
        print "client < $line";
        sleep 1;
        ping($sock);
    }
}

sub ping {
    my ($fh) = @_;

    print "client > ping\n";
    print {$fh} "ping\n";
}

sub pong {
    my ($fh) = @_;

    print "server > pong\n";
    print {$fh} "pong\n";
}