Maybe my brain has been damaged by lack of caffiene, but it seems to me the other monks are answering a different question than the one you asked.

Current Situation

You have a web page which displays a form. When the user completes the form, it is sent to a CGI script, which transmits the information to a third party.

Of course, other possibilities exist; for example, that your web site displays a page containing a form, but when the Submit button is pressed, the data goes tot he third party, without ever coming to your site.

Desired Change

You wish the data to be saved at your system.

Trivial Solution

Is it possible to configure Apache so that the POST arguments are made part of the log? Is it possible the information already is being logged?

Less Trivial Solution

When the button is pressed, a CGI script is invoked. Modify the script so it saves the data before sending it off to the third party.

Somewhat Tougher Solution

If the data is going directly to the third party, modify the web page containing the form, so it invokes your CGI script instead of someone else's. Your script can save the data, then re-direct the user to the remote script, or it can invoke the remote script "secretly" and then pass the return values to the user.