Well, I havn't taken a detailed look at the module yet, but I'm going to comment anyway... about it's name, which is downright abysmal.

A "postal code" is a generic term, refering to similar schemes that exist in many countries. The term for the purticular scheme of codes that exists in the US is ZIP code. The module should be named Geo::PostalCode::US or Geo::ZIPCode, not Geo::PostalCode.