Rules to remember:
1. Please review your documentation before asking a question.
2. Please provide enough information to answer your question when asking.

I note those two rules not to be hostile, but to show why it is difficult to give you a specific answer to your question. It sounds like you want to do is to set a variable (in this case, "bar") before calling the CGI (in this case, "foo.cgi"). Since you did not include the type of webserver (#2, above), I cannot tell you exactly how this could done, although likely the documentation for the server can do so (#1, above). However, an example of documentation providing such an answer can be found in the Apache 1.x webserver documentation at http://httpd.apache.org/docs/1.3/howto/ssi.html, specifically in the section entitled "Setting variables".

HTH.