According to lib:
in reply to "use lib" and architecture-specific paths
For each directory in LIST (called $dir here) the lib module also checks to see if a directory called $dir/$archname/auto exists. If so the $dir/$archname directory is assumed to be a corresponding architecture specific directory and is added to @INC in front of $dir.
Does that not actually work under some circumstances? Does it require the auto subdir to exist? I confess to never having developed any arch-specific modules so I haven't tested it.