use strict;
use warnings;
use IO::Socket::INET;

my $PORT = 53123;
my $num  = 0;

my $sock = new IO::Socket::INET(
    LocalHost => '127.0.0.1',
    LocalPort => $PORT,
    Proto     => 'tcp',
    Listen    => 5,
    Reuse     => 1,
) or die;
binmode $sock;

while ( my $client = $sock-> accept ) {
    my $str = do { local $/; <$client> };
    die "$num\n" unless $str eq '123';
    print "$num\n" unless $num ++ % 1000;
}