You want to limit the number of lines to the shortest workable number. Since you problem is with the server hanging, I reduced your script to the minimum necessary.

#!/usr/bin/perl 

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

my $sock = IO::Socket::INET->new(
           Listen    => 5,
           LocalHost => 'localhost',
           LocalPort => 12400,
           Proto     => 'tcp',
           Timeout   => 15,
           Blocking  => 0) 
or die "Couldn't create socket: $!\n";
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Blocking is default unless Blocking is set to 0, which is nonblocking.