Foxcub's method of using frames works, but doesn't completely obscure the fact that the page comes from the other domain (the recipient need only View Source to see what's going on).

Another approach is for your CGI, after it's done whatever processing it needs to, to fetch (using LWP) whatever page you want to feed the browser, and pass that page through to the browser. Something like:

  use LPW::Simple;
  ...
  print "Content-type: text/html\n\n";
  print get("http://www.yahoo.com/");
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