For 1991, Martínez improved. For the fourth year in a row, he was the Opening Day starter, pitching against the Pittsburgh Pirates at Three Rivers Stadium. He threw seven innings of work while allowing one hit and no runs with five strikeouts and two walks as the Expos won 7–0. It was his first and only Opening Day win with the Expos. He went 14–11 with a 2.39 ERA (a career low) in 31 games and 222 innings of work. He had nine complete games while having five shutouts, the latter being a career high. July proved to be a highlight He had 123 strikeouts with 62 walks while being named to the All-Star Game at SkyDome in Toronto on July 9. Martínez pitched the third and fourth inning for the NL, allowing three runs on four hits, including a home run from Cal Ripken, Jr. He received the loss as the American League held on to win 4–2. Nineteen days later, on July 28, Martínez pitched the 13th perfect game in Major League Baseball history, pitching against the Los Angeles Dodgers. He was the first Latin American–born pitcher to pitch a perfect game. Martínez struck out five batters and threw 96 pitches, 66 for strikes. He finished the month of July with a 2–1 record in five games started while named NL Pitcher of the Month. He finished with the lowest ERA in the league, beating out José Rijo, who finished with a 2.51 ERA. He was the first Expo since Steve Rogers in 1982 to win the ERA title.