At the end of February 2011 a new zip code system for whole Peru was introduced. While in the past only Lima and Callao had zip codes, now all cities and smaller settlements in the country are included in the new system. The new zip codes have 5 digits; the ones for Lima start with a 15, for Callao with 07. As the zip codes in Lima and Callao are no longer linked to the district, a search on the website of the Ministry of Transportation and Communication (MTC) is unavoidable.
Unfortunately the website of the MTC frequently doesn't work and the link to the postal codes changes also permanently. For that reason we do not have a direct link to this information, but you might it under their website under "codigos postales".
However we managed to get the latest maps for the new postal codes in Lima & Callao (have a look at PDF Documents section at the end of this article).
Up to now we personally still continue using the very simple previous postal codes for all our correspondence without any problems. Due to all this confusion we recommend sticking to the old codes and we update this information as soon the change has been implemented!