As of v1.09, Gabor Szabo cleaned up all of the problems I complained about 6 years ago in v1.08, so I give this simple library full marks. It could probably be implemented more efficiently with a hash of behaviors rather than if/elsif/elsif/elsif/.../else, but that's just an implementation detail.
This module is a straightforward RPN calculator implementation. The code is very readable, but the packaging is abysmal. The author simply tarred and uploaded a single .pm file, lacking a license, Makefile.PL, or a README.
If the packaging were polished, I would probably give this module a rating of 5.