server:
use strict;
use IO::Socket;
use threads;

$|++;
my $server = IO::Socket::INET->new(LocalPort => '5123',Reuse => 1,Listen => 5)
or die "Could not create server: $!\n";

while (my $client = $server->accept()) {
	threads->create(\&talk_with_one_client, $client)->detach();
}

sub talk_with_one_client {
	my $client = shift;
	while (my $line = <$client>) {
		print $line;
		chomp $line;
		print $client $line + 1, "\n";
	}
}

client:
use strict;
use IO::Socket;
$|++;
my $socket = IO::Socket::INET->new(PeerAddr => "localhost", PeerPort => '5123', Proto => "tcp") 
or die "Could not create client: $!\n";
print $socket "1\n";
while (my $line = <$socket>) {
	print $line;
	chomp $line;
	sleep 1;
	print $socket $line + 1, "\n";
}