The question is, how to know if I'm on Darwin or on Mac OS X using pure Perl!

Just to re-iterate Mac OS X is Darwin, just like Redhat and Slackware are both Linux. You cannot be on a Mac OS X system and not be on Darwin.

If you want to know whether your R binaries are in /Library/Frameworks then knowing the difference between a Mac OS X box and any other Darwin box isn't going to help you.

Frameworks are a Darwin way of bundling stuff together. A non-Mac OS X box running Darwin is also likely to have R in /Library/Frameworks.

(Equally, a Mac OS X box may not have the R binaries in /Library/Frameworks because whoever installed it compiled it from source and chucked it somewhere else.)