Mills was sent down to Triple-A Las Vegas on Wednesday following the Blue Jays' 8-3 win over the Mariners. Toronto said at the time that a corresponding move would be made on Thursday's off-day, and Enrique Rojas of ESPN Deportes reported that Perez was promoted.
Perez has made one appearance in Triple-A this season, a five-inning start in which he allowed four runs on as many hits. The 26-year-old from the Dominican Republic has never appeared in the Majors.
Mills, also 26, was with the big league team for only two games, and he did not make an appearance. He has nine career Major League appearances over the last two seasons.
The Blue Jays start a four-game series with the Red Sox in Boston on Friday.