proc_open: Extending file descriptor numbers to enable "status" feedback from a Perl script

PHP's proc_open manual states:

The file descriptor numbers are not limited to 0, 1 and 2 - you may specify any valid file descriptor number and it will be passed to the child process. This allows your script to interoperate with other scripts that run as "co-processes". In particular, this is useful for passing passphrases to programs like PGP, GPG and openssl in a more secure manner. It is also useful for reading status information provided by those programs on auxiliary file descriptors.

What Happens: I call a Perl script in a PHP-based web application and pass parameters in the call. I have no future need to send data to the script. Through stdout [1] I receive from the Perl script json_encoded data that I use in my PHP application.

What I would like to add: The Perl script is progressing through a website collecting information depending on the parameters passed in it's initial call. I would like to send back to the PHP application a text string that I could use to display as a sort of Progress Bar.

How I think I should do it: I would expect to poll (every 1-2 seconds) the channel that has been setup for that "progression" update. I would use Javascript / jQuery to write into an html div container for the user to view. I do not think I should mix the "progress" channel with the more critical "json_encode(data)" channel as I would then need to decipher the stdout stream. (Is this thought logical, practical?)

My Main Question: How do you use additional "file descriptors?" I would image the setup of additional channels to be straightforward, such as the 3 => ... in the below:

$tunnels = array(   
   0 => array('pipe', 'r'),     
   1 => array('pipe', 'w'),    
   2 => array('pipe', 'w'),    
   3 => array('pipe', 'w')        
);

$io = array();
$resource = proc_open("perl file/tomy/perl/code.pl $param1 $param2 $param3", $tunnels, $io);

if(!is_resource($resource)) {
    $error = "No Resource";
}

fclose($io[0]);

$perlOutput = stream_get_contents($io[1]);
$output = json_decode($perlOutput);

$errors = stream_get_contents($io[2]);
print "$errors<p>";

fclose($io[1]);
fclose($io[2]);

$result = proc_close($resource);

if($result != 0) {
    echo "you returned a $result result on proc_close";
}

But, in the Perl script I simply write to the stdout like:

my $json_terms = encode_json(\@terms);
print $json_terms;

If my understanding of setting up an additional channel is correct (above, the 3 => ...), then how would I write to it within the Perl script?

Thanks