Isn't that generally the very function web servers were created to perform? :)

Take a look at open and print to see how to do this with perl, though I really am at a loss to find a reason anybody wants to do this with a CGI. I mean... really... that's what web servers do. They serve files over the web.

Update: On re-reading, I think what you want to do is include a text file inside another web page. This seems like a bit more reasonable use for a CGI. Apologies for the assumption -- reading about open and print should steer you in the right direction.