Rusty perl question

Hi all,

I used to know the trick, I think, but can't remember. I want to code my 
application to be smart about whether it's on a system that can use 
mod_perl2/apreq, or if I have to use CGI. I 've code my app to use CGI 
successfully, but would like to keep the code that I worked to get apreq 
working as well. I would like for my application to be smart enough to 
say use apreq if it's there, or use CGI if it isn't. What's the magical 
trick to toggle the 'use libfoo' or 'use libfee'? Kindof like #ifdefs in 
C...

Thanks!

Patrick