Since then, Humber's season has been pretty awful. In 10 starts since the perfecto, he has a 7.47 ERA and has allowed 12 home runs in 53 innings.
This could be an explanation for that performance, from a White Sox news release:
Prior to tonight’s game vs. Milwaukee, the Chicago White Sox placed right-handed pitcher Philip Humber on the 15-day disabled list (retroactive to June 17) with a right elbow flexor strain and purchased the contract of right-hander Brian Bruney from Class AAA Charlotte.
There's no indication how long Humber has been dealing with this issue, but it very well could have caused some of the rough outings. Bruney is pretty much exclusively a relief pitcher (one career start in 272 appearances), so the White Sox will have to figure out someone else to take over Humber's rotation slot.