I can see how each message is separate, but when I create a socket to broadcast, I have to specify it's for broadcasting:

$sender = IO::Socket::INET->new(
    PeerAddr      =>  inet_ntoa(INADDR_BROADCAST),
    Broadcast     =>  1,
    PeerPort      =>  '7071',
    Proto         =>  'udp'
) or die "cannot connect: $!\n";
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So once I do this, doesn't that change the nature of the socket? I don't see a method in IO::Socket::INET or IO::Socket that lets me change the Broadcast flag to 0 after I send a broadcast. I've been trying with sockopt(), but it won't set or change any values.

No, you don't have to connect to INADDR_BROADCAST.

use strict;
use warnings;

use IO::Socket::INET qw( INADDR_BROADCAST inet_ntoa pack_sockaddr_in u
+npack_sockaddr_in );
use threads          ;#qw( async );

my $client = async {
   my $s = IO::Socket::INET->new(
      Proto     => 'udp',
      LocalPort => 7071,
      Broadcast => 1,
   )
      or die("Can't create socket: $!\n");

   defined( my $peer = $s->recv(my $msg, 64*1024, 0) )
      or die("Can't recv: $!\n");

   my ($port, $addr) = unpack_sockaddr_in($peer);
   $addr = inet_ntoa($addr);
   print("Received $msg from $addr:$port\n");

   $s->send("Pong!", 0, $peer);
};

{
   my $s = IO::Socket::INET->new(
      Proto     => 'udp',
      Broadcast => 1,
   )
      or die("Can't create socket: $!\n");

   $s->send("Ping!", 0, pack_sockaddr_in(7071, INADDR_BROADCAST));

   defined( my $peer = $s->recv(my $msg, 64*1024, 0) )
      or die("Can't recv: $!\n");

   my ($port, $addr) = unpack_sockaddr_in($peer);
   $addr = inet_ntoa($addr);
   print("Received $msg from $addr:$port\n");
}

$client->join();
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Received Ping! from 10.0.0.6:49853
Received Pong! from 10.0.0.6:7071
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I have to specify it's for broadcasting [...] doesn't that change the nature of the socket?

I think it simply allows it to broadcast and receive broadcasts.

What catches my eye is that you chose to connect to INADDR_BROADCAST. Connecting a UDP socket (i.e. specifying PeerAddr) means you can only receive from the address to which you connect. What does it mean to connect to INADDR_BROADCAST? ( Can only receive broadcasts ) Do you have to connect to INADDR_BROADCAST? ( no ) Will test tomorrow.

I don't see a method in IO::Socket::INET or IO::Socket that lets me change the Broadcast flag to 0 after I send a broadcast.

Since Broadcast=>1 does

$sock->sockopt(SO_BROADCAST,1)
[download]

the approach would be

$sock->sockopt(SO_BROADCAST,0)
[download]

But again, I don't think this is necessary.

Once I studied your code and altered mine and got tests working, I found other issues with xinetd and UDP, so, in the long run, I had to send the "Where are you" message on one port, then the server had to reply on a different port. I tried using the same port for the server, but I couldn't open a 2nd socket there using the same port.

While the solution didn't come directly from what you showed me, your code helped me fix what I had wrong there and let to me finding the root issue.

Thanks!