Some alternatives to consider, though...

1. The browser shows the URL of my progrma rather than the external CGI program.

Skip a Perl intermediary altogether, and use JavaScript to do your param munging with an onClick/onSubmit.

Sorry. not really an option. Many people (very sensibly) diable JavaScript.

2. The relative URLs within the displayed content are all broken as the browser is interpreting them as relative to my program.

Have your script include a <base href="http://url.to/the/external/CGI" /> in it's output so that relative URLs are correctly interpreted by thr browser.

As I understand it, a "Content-Base:" header will work just as well and will be easier to add. I'm looking at that option now.

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg