Sometimes it is needed to pass additional arguments when creating a Makefile. For example when installing Geo::IP module you don't have gcc and you want to run:
perl Makefile.PL PP=1
or you want to pass an alternative library path
perl Makefile.PL LIBS='-L/path/to/library'
this is going to be part of 1.7
Implemented in a49e8f8 shipped as 1.6191-TRIAL.