[yocto] Help me understand Openembedded variable expansion

[Matt Schuckmann]

So there is something I just don't get about how variables get created, expanded, interpreted, etc in Bitbake

For example I'm trying to copy some of the techniques in the Poky Tiny distribution to reduce my image size and build time. 
One of the fragments is this one. 

# The mtrace script included by eglibc is a perl script. This means the system
# will build perl in case this package is installed. Since we don't care about
# this script, remove the dependency from here.
RDEPENDS_${PN}-mtrace_pn-eglibc = ""

I put this in my distribution .conf file and I doesn't seem to do anything perl is still getting built. 
Of course it doesn't help that I don't understand what does, especially in the context of the distribution .conf file. 

I also see hints in the code in debug output that the machine, image, or distribution names can automatically get appended to paths, or variables but I don't really understand how that works. 

I'm not even sure how to phrase a question around this, immediately I'd like to figure out why Perl is still getting built but ultimately I'd like to understand the variable rules better so I that I can better take advantage of this power tool. 

Thanks,
Matt S.