Atlee Hammaker pitched a perfect game for seven innings yesterday in San Francisco before yielding Johnny Bench's leadoff single in the eighth, then finished with a two-hitter as the Giants beat the Cincinnati Reds, 3-0, in the first game of a doubleheader. The Reds won the second, 12-3, after a nine-run first inning.

Bench lined a clean hit to left on the first pitch in the eighth. The next batter, Jeff Jones, flied to center and Alan Knicely grounded into a double play started by shortstop Johnnie LeMaster. Hammaker, 25, who attended Mount Vernon High School in Alexandria, also gave up a one-out single to Dann Bilardello in the ninth, but faced only 28 batters, one more than the minimum.

Hammaker (1-1) struck out 10, his major league high, beating another young left-hander, Joe Price (1-1), who allowed only five hits.

"It was fun. I'd like to try it again," said Hammaker, who was cheered loudly by a Cap Day crowd of 37,094 as he retired batter after batter with his fast balls and sliders. "He (Bench) felt confident against me. I could tell that. His first time up he hit the ball well."

"The pitch I got the hit on was the same pitch I hit in the second inning, a slider up over the plate," Bench said. "I was sort of guessing with him in the eighth. I was confident I'd get a pitch to hit, and I was going to hit it.

"If he'd been working on just a no-hitter and not a perfect game, he might not have thrown that on the first pitch."

Hammaker said Bench's hit caused him no great disappointment. "I was able to forget about the no-hitter and start thinking about the game. We needed to win one badly." The Giants had lost four straight.

In the second game, the Reds sent 13 batters to the plate against three pitchers in the opening inning. Tom Foley's first major league hit, a bases-loaded double off Mike Chris, drove in three runs.