Perl: PayPal IPN Response is Invalid -- returns HTTP::Response=HASH(....

I'm integrating PayPal into our web store.
We send people to PayPal where they pay.
Then PayPal Sends us an IPN (Instant Payment Notification), that contains data about the payment.
We have a 'Listerner' program that receives the IPN.
When we receive the IPN, we are supposed to POST the data back to PayPal for validation.

PayPal Provided some code in Perl for this.
I used the code provided by PayPal, and integrated it into my page.
 
Here is some of the code in the 'Listener' program:
--------------------------------------------------------------------------
use LWP::Protocol::http;
use LWP::UserAgent;

$payPalTestMode = 1;

read(STDIN, $buffer, $ENV{'CONTENT_LENGTH'});
$query = $buffer;

&writePayPalIpnLog("IPN RECEIVED: ".$query);
     
$query .= '&cmd=_notify-validate';

# post back to PayPal system to validate
$ua = new LWP::UserAgent;
if ($payPalTestMode){
$req = new HTTP::Request 'POST','https://www.sandbox.paypal.com/cgi-bin/webscr';
}else{
  $req = new HTTP::Request 'POST','https://www.paypal.com/cgi-bin/webscr';
}
$req->content_type('application/x-www-form-urlencoded');
$req->content($query);
$res = $ua->request($req);
 
&writePayPalIpnLog("Validation Response ".$res);

-------------------------------------------------------------------------------

From my log file, I can see that I received the IPN from PayPal.
And I also log the response I get back from the POST that is sent.
The response I get back is like: HTTP::Response=HASH(0x9251de4)
(It should be VERIFIED or INVALID.)

I used PayPal's IPN simulator to send an IPN to my Listener program.
If I Post the response back to PayPal, then the following message is displayed by the IPN simulator:
"IPN delivery failed. HTTP error code 502: Bad Gateway"

If I do not POST back to PayPal, then the IPN simulator displays:
"IPN successfully sent."

So it appears that something about the POST is not working right. But I don't know what it could be.

I'll appreciate any help in this.