Ok. Once I wrap my stuff, how do I get the parameters from GET? How do I build a new URI from an existing one, changing only one or two params? I've read through some 500 pages in the past two days and I'm getting nowhere. I know tons about how to secure a mod_perl server, but I can't find out how to USE the bloody thing!

Walk away. Seriously, just walk away for a few hours/days. This stench of despair is not good. http://perl.apache.org/docs/2.0/user/coding/cooking.html

 use CGI::Cookie ();
  use Apache::RequestRec ();
  use APR::Table ();
  
  use Apache::Const -compile => qw(REDIRECT);
  
  my $location = "http://example.com/final_destination/";
  
  sub handler {
      my $r = shift;
  
      my $cookie = CGI::Cookie->new(-name  => 'mod_perl',
                                    -value => 'awesome');
  
      $r->err_headers_out->add('Set-Cookie' => $cookie);
      $r->headers_out->set(Location => $location);
      $r->status(Apache::REDIRECT);
  
      return Apache::REDIRECT;
  }
  1;
[download]

The first argument to your handler is a Apache::RequestRec object. Now go read some documentation to see what you get with Apache::RequestRec.