Box score data doesn't exist for the first four perfect games, reducing our sample size to 17 games. My primary unit of measure is the Game Score (GSc), a metric developed by Bill James to determine the effectiveness of a starting pitcher. The measure starts at 50 and adds:
1 point for each out, 3 for each complete inning, 2 for each inning after the 4th and 1 for each strikeout
-2 for each hit, -4 for each earned run, -2 for each unearned run and -1 for each walk
I strongly suggest magnifying the browser to see the numbers, but it would appear that Humber's next outing after his perfect game WAS the worst follow-up game after a perfect game. He gave up 9 runs in 5 innings, and the next-closest would be Catfish Hunter's follow-up in 1968. In two examples, I had to look at games in the next season, but the numbers show what has been known for as long as baseball has been played--one game's results have no bearings on the next.