use strict;
use warnings;

use Socket qw( INADDR_LOOPBACK PF_INET SOCK_STREAM SOMAXCONN pack_sockaddr_in );

my $port = 9999;

socket(my $socket1, PF_INET, SOCK_STREAM, 0)
   or die("socket[1]: $!\n");

bind($socket1, pack_sockaddr_in($port, INADDR_LOOPBACK))
   or die("bind[1]: $!\n");

socket(my $socket2, PF_INET, SOCK_STREAM, 0)
   or die("socket[2]: $!\n");

bind($socket2, pack_sockaddr_in($port, INADDR_LOOPBACK))
   or die("bind[2]: $!\n");      # This fails (Address already in use)

listen($socket2, SOMAXCONN)
   or die("listen[2]: $!\n");

listen($socket1, SOMAXCONN)
   or die("listen[1]: $!\n");