pg is on the right track when he says to use two sockets. You don't want to use the same socket to both send and receive over UDP. The code from the cookbook is for TCP sockets and won't work with UDP. If you want to use UDP, here is your code changed to use two sockets:

#!/usr/bin/perl
# biclient - bidirectional forking client   perl_book\cookbook\ch17_11
+.htm
use IO::Socket;
use English;
use strict;

my ($lport, $host, $port, $kidpid, $sock, $line, $msg, $MAXLEN);
$MAXLEN = 1024;
$|++;

unless (@ARGV == 3) { die "usage: $0 listen_port dest_host  dest_port\
+n" }
($lport, $host, $port) = @ARGV;

# create a udp connection to the specified host and port
my $in = IO::Socket::INET->new(Proto     => "udp",
                              LocalPort => $lport )
       or die "can't connect to port $lport on localhost: $!";

my $out = IO::Socket::INET->new(Proto     => "udp",
                              PeerAddr  => $host,
                              PeerPort  => $port)
       or die "can't connect to port $port on $host: $!";


print STDOUT "[Listening on $lport, Connected to ($host:$port)]\n";
# Thelonius notes: Actually, with UDP, we 
# are NOT CONNECTED to ($host:$port).
#  However, our messages will be sent there

# split the program into two processes, identical twins
die "can't fork: $!" unless defined($kidpid = fork());

if ($kidpid) {
    print STDOUT "Parent $PID started\n";

    # parent copies the socket to standard output
    while ($in->recv($msg, $MAXLEN)) {
        print "\nReceived from ($host:$port) \"$msg\" \n";
    # Thelonius notes: You do not know where
    # the message came from unless you use
    # recvfrom.
    }
    # This loop will never exit.
    kill("TERM" => $kidpid);        # send SIGTERM to child

} else {
    print STDOUT "Child $PID started\n";
    print $out "Child $PID started";

    # child copies standard input to the socket
    while ($line = <STDIN>) {
        chomp($line);  #print STDOUT "You entered: $line\n";
        print $out $line;
    }
    # You might want to kill parent if you exit.
}
exit;
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