Uh, because the Perl installation requires them?
The way that Perl organizes the libraries is designed as a "search path" - run this command from the shell:
perl -e 'print "@INC\n";'
and you will get a list of directories that show the order of searching for modules, libraries and executables when Perl is asked to load them. The '.../darwin' directory occurs before the parent directory to allow libraries that are specific to the architecture to load before an equivalent one that might be machine independent (but slower).
Now, what was your question again?