I'm not an expert, but my understanding is that Peru has a dry season that lasts approximately between April and October.   During this season it is pretty difficult to get rainfall (not unheard of!) because the waters off the coast of Peru are colder.  Colder water inhibits rainfall.  So, El Nino means that cold water near Peru is warmer than normal, but it is usually still to too cold to cause rainfall.  This will change as we get closer to November and is why Peru recently declared a preemptive state of emergency.