To be both the client and the server, set up the program as a server. This way you're always ready to accept a transmission. First you establish the connection:

$server = IO::Socket::INET->new(
  Listen    => 5,
  LocalPort => 4000,
  Proto     => 'tcp',
  Reuse     => 1,
  Type      => SOCK_STREAM
) or die "Can't create server socket: $!";
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Then you wait for transmissions:

while ($client = $server->accept()) {
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Within this while loop, you can both accept transmissions, and respond to $client with something like:

print $client $ack;
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Hope this helps.