I think he is just not used to pitching this much.
Perez was sidelined for part of 2018, but he threw 185 innings in 2017 and 198 in 2016 and did just fine in August/September those years. (Plus 165 and 167 combined majors/minors innings back in 2012 and 2013, respectively.)
Even in 2018, despite missing May and most of June, he had 102 innings total and likely could have had 117 or so if not for his bullpen demotion in September.
Seems doubtful he's seeing effects of overuse before reaching 65 innings in 2019.
He's also had 5, 6, and 5 days of rest before each of his last 3 starts. Since he joined the rotation, only 4 of his 9 starts have been on the standard 4 days rest.