I'm not sure about your question regarding writing to STDIN, but does the following work for you in a POST?

#!/usr/bin/perl -wT
use strict;

my $buffer;
read (STDIN, $buffer, $ENV{'CONTENT_LENGTH'});
print "Content-type: text/plain\n\n";
print $buffer;
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Also here is a script you may find interesting.

#!/usr/bin/perl

# Description:  Takes all CGI parameters (both GET and POST data) 
# and builds a query string to send as a GET request.

use CGI::Simple; # or use CGI;
use URI::Escape;

my $CGI = CGI::Simple->new; # or CGI->new;

my $query = join(
  ';', map {
     uri_escape($_) . '=' . uri_escape($CGI->param($_))
  } $CGI->param
);

# now we can send a GET rquest somewhere
$CGI->redirect('http://my.server.com/script.pl?' . $query);
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