The club had spent the past few days deciding which young pitcher would slide into that slot.
Perez, 21, was called up Tuesday to give the club an additional bullpen arm after Justin Grimm lasted just one inning (plus two batters) Monday, forcing Michael Kirkman and Mark Lowe into long relief outings.
Perez took Colby Lewis' place on the 25-man roster when Lewis was put on the disabled list with tendinitis in his right forearm.
"I hear it's a little different," Perez said Friday of starting. "I will try and do my best with my pitches and keep it low and do the best that I can."
Perez made his big league debut in Wednesday's 13-9 win over the Detroit Tigers, giving up four runs (one earned) on two hits with a walk and a strikeout in 2/3 of an inning.
"I have to work a little bit on my control because I left some pitches up," Perez said after Wednesday's game. "I trust what I can do. I trust my pitches and I trust my teammates."
At the very least, the outing allowed Perez to get out some of the jitters.
"He probably had some nerves working," manager Ron Washington said Wednesday. "I'd certainly like to see him come and get his three outs, but that just didn't happen. At least he got his chance to get on the mound. He tried to keep the ball down. Once again, his opportunity was against a pretty good hitting offense. At least he got a chance to get in the game, and we'll see where he goes from there."
Grimm remains available in the bullpen and can give the club multiple innings, if needed.
Perez earned the opportunity to join the big league club in part because he showed improvement with his fastball command in his past two starts at Triple-A Round Rock, winning both.
Perez, who had struggled for most of the season, allowed just two runs in 16 innings, with opponents going 4 for 50 (.080).
Overall, Perez has posted a 5-5 record with a 4.59 ERA in 15 Triple-A starts with 49 strikeouts and 38 walks.