The simple answer is "don't". It will confused the heck out of everybody reading the code.

That said, if you understand it's a stupid idea, and you're convinced that there's no betters solution (if you told us what your deeper problem is, maybe we could find one for you?), there are still two approaches:

1) edit the existing CGI.pm and add your changes. Optionally with copying to another location first.

2) you can try to remove CGI.pm from %INC, remove the paths where the current module was found from @INC, and then use CGI; and have fun.