A new poll out today by Datum and published in Peru 21 has Keiko up half a point from last Sunday. Here's the chart that shows today's numbers (red) and last weekend's poll (grey).
Note that the Datum/Peru 21 combo is known to be anti-Humala in nature and more reasonable pollster Ipsos/Apoyo had Keiko's lead at under three points last weekend. With that said, the trend shown here can't be good for Humala. The latest Datum poll was taken on May 22nd, asked 1214 people on a national urban/rural basis and has a margin of error of +/- 2.8%. Data from here.