At the end of February 2011, they introduced a new zip code system for whole Peru. 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 got the latest maps for the new postal codes in Lima & Callao (have a look at Attachment section at the end of this article).