I would have liked to use CGI::url ( -absolute => 1, -query_string => 1 ) but this returns the POSTed params instead of the Query String.

If I understood you correclty, you are processing a POST request with POST data and a query string and like to retrieve just the URL parameters. In that case the following should work for you:

#!/usr/bin/perl

use strict;
use warnings;
use CGI;

my $q = CGI->new();

sub url_query_string {
  my ($q) = @_;

  # get the names of all URL parameters
  my @params = grep { length($_) } $q->url_param;

  return (@params > 0 ? '?' : '').join('&',map {
    # $q->url_param returns unescaped values so we have to escape them
    # again
    sprintf("%s=%s",$_,CGI::escape($q->url_param($_)))
  } @params);
}

print $q->header('text/plain'),
  $q->url(-absolute => 1).url_query_string($q),"\n";
[download]

Testing it with curl (assuming the base URL is localhost:8080/cgi-bin/test.pl):

$ curl -d foo=bar localhost:8080/cgi-bin/test.pl
/cgi-bin/test.pl
$ curl -d foo=bar localhost:8080/cgi-bin/test.pl?msg=hello%20world
/cgi-bin/test.pl?msg=hello%20world

Hope that helps.