#!/usr/bin/perl

use IO::Socket;
use strict;

my( $socket,$ret );
my $maxlen  = 1024;
my $timeout = 5;
# my $data = "\x02H\x04";  # getVersion <STX>CMD<EOT>
my $data = "\x02LS9F\x04"; # getSerial <STX>CMD<CHECKSUM><EOT> 
$socket = IO::Socket::INET->new (
   PeerAddr  => '192.168.1.16',
   PeerPort  => 10001,
   Proto     => 'tcp'
) or die "ERROR in + Socket Creation : $!\n";
main::printdata('Client sends',$data );
$socket->send($data) or die "send: $!";
$socket->recv( $ret,$maxlen );
main::printdata('Server responded',$ret );
$socket->close();
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How do I increase the verbosity level to find out what's going wrong here?

Item 1 in the Basic debugging checklist suggests to use warnings which you are not doing here. Enable that for starters.

$socket->recv( $ret,$maxlen ) or die "Fail: $!";

The docs for recv say:

Returns the address of the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an error, returns the undefined value.

But your statement only tests for truth rather than definedness. Try this instead, perhaps:

$socket->recv( $ret,$maxlen ) // die "Fail: $!";