I created a custom team and edited all stats for every player to be zero. I then chose the standard Sirloins team to play as and set the innings to 9 for the requirement. For the first inning, you will just need to score 1 run, which should be easy, as all of the stats for the other team are at zero and they have virtually no fielding or speed skills, and this is all you need to win since you will not be giving up any hits/walks/runs. Once you have the necessary run, just bunt (B) to complete the innings more quickly.
Just to be safe, pitch the corners using a variation of fastball and slider/curveball, depending on the pitcher. The players will be easily 'fooled' on most pitches and most of your outs should come from strikeouts. It is very rare for them to 'anticipate' the pitch and make contact, but since they have little hitting power, these should turn into easy fly-balls or in-field grounders.
A perfect game is where the pitcher does not give up any hits or walks. If anyone on the opposing team reaches base, for any reason, the perfect game is invalid and you will need to restart. Just be careful when you are pitching and it should only take about 20 minutes per complete game.