A flood of new umpires will patrol games this season with 10 rookies replacing retiring veterans across Major League Baseball, MLB vice president Michael Hill said last month. It will be the big leagues’ largest first-year class of umps in more than 20 years.

The newbies, ranging in age from 31 to 40, are joining a league that is rapidly evolving its rules amid a particularly anxious future for umpires.

This season, MLB will for the first time introduce a pitch clock, shift limits and larger bases, the league’s most seismic set of rule changes in nearly 50 years. And, for the first time this season, every Triple-A ballpark will employ an automated strike zone, so-called robo umps, after debuting the system last year.

MLB typically rotates a cast of 19 umpire crews in a season, each including four umpires who work the entirety of a series before moving along to another city. Those crews will see a thorough shakeup.

Tom Hallion was the longest-tenured among those umps who are retiring, with 30 years of experience. And Ted Barrett, who worked 26 seasons in the majors, is the only ump in baseball’s history to have worked two perfect games. Barrett was on the field when the Yankees’ David Cone pitched a perfecto against the Expos in New York in 1999 and when the Giants’ Matt Cain threw one against the Astros in 2012.

The other umps who are retiring: Jerry Meals (26 years of experience), Marty Foster (24 years), Greg Gibson (24 years), Jim Reynolds (23 years), Tim Timmons (23 years), Bill Welke (23 years), Sam Holbrook (22 years) and Paul Nauert (22 years).