Well, running the command "ppm install" would actually download the module, then automatically install the module to the lib folder in Perl for you(if the module was available in the specific perl distribution repository), hence it circumvents the need to download and then navigate to that folder and install a module and all the steps involved. Check ppm.

Now you downloaded the RRDs.ppd yourself, you need to manually run its MakeFile.pl first and compile it through so you would need the nmake.exe utility to support the manual installation steps on Windows on a similar fashion to that in *nix based systems (i.e 'make', 'make test' and 'make install' would become 'nmake', 'nmake test', 'nmake install' respectively.)...

check:
A guide to installing modules for Win32.
A Guide to Installing Modules