Yesterday, in the Mets’ 3-1 win over the Yankees at Shea Stadium, starting pitcher Oliver Perez did something he hasn’t done once all season — or, strictly speaking, two things. His main achievement, aside from pitching seven innings and giving up just one run in a badly needed win, was to pitch an entire game without walking anyone. Perez hasn’t done this since last September 22, and before that August 5, so congratulations are in order. This was, though, just a consequence of a more noteworthy achievement: For the first time this year, the schizophrenic left-hander threw twice as many strikes as balls in a game.

If it isn’t the sort of thing that ends up on the backs of baseball cards, strike percentage is still one of the most important numbers in the game. Every strike a pitcher throws is a ball unthrown, and each of those makes him less likely to end up in a hitter’s count or putting someone on first base. This is especially important for a pitcher like Perez, who relies on a slider that breaks out of the strike zone and is thus much more effective when hitters are trying to protect the plate. In his career, batters have been hitting .256 with a .420 on base average after passing through a 1-0 count; those figures drop to .210 and .267, respectively, after 0-1.

As Hall of Fame manager Earl Weaver wrote in his classic book “Weaver on Strategy” when explaining why he droned on endlessly to his pitchers about throwing strikes, “You keep talking about it because it’s so important.” (He would doubtless have put the point more colorfully had he been forced to manage Perez.)

Going into yesterday’s game, Perez had thrown 59% of his pitches for strikes. Basically, this is why he’s been having a lousy year. By contrast, Johan Santana has thrown twice as many strikes as balls in 10 of his 17 starts for the Mets, and overall he’s thrown 65% of his pitches for strikes. This may not seem like an enormous difference, but throwing 10% fewer strikes than Santana is much of the reason why Perez makes 10% what his teammate does.

The encouraging news for Mets fans is that yesterday wasn’t an isolated incident. Over his last four starts, Perez’s results have been typically erratic — he’s wrapped gems against the Yankees and Rangers around a mediocre start against the Angels and an awful one against the Mariners — but he’s thrown at least 63% of his pitches for strikes in every game. Before this stretch, he hadn’t broken 60% in two straight games all year, even during his brilliant early April, when he allowed no runs in three of his first four starts.

If he keeps this up, results will follow. In the eight games in which he’s thrown less than six of 10 pitches for strikes, he’s giving up 8.92 runs per nine; in the other eight, 3.75. The difference really is that simple, which is why manager Jerry Manuel was right to say, after yesterday’s game: “If we can get Ollie to be consistent, which has been the dilemma for him his whole career, we will have an excellent pitcher, no question.”

As Weaver knew, though, these ifs don’t count for much. “Obviously,” he wrote, “pitchers don’t try to walk batters. Most of the time it’s a matter of confidence.” This is a crucial and often-overlooked point. Perez, like all pitchers who don’t throw enough strikes, is in fact aware that it’s better not to throw bad pitches and get in bad counts, and that he’d make more money if he were more consistent. If he were more confident and so never tried to get too fine with his pitches, and if he had better mechanics, he’d be more consistent, and then he’d be an excellent pitcher. But he isn’t.

The problem with Perez isn’t that he’s bad — he isn’t — but that he sometimes seems to be better than he is. He’ll dominate the Yankees, or throw a decent number of strikes for several games in a row; suddenly all the ifs will seem less iffy and he’ll stand on the mound looking as if a lightning bolt has hit him and unveiled a Cy Young winner in all his glory. This makes it disappointing when he goes back to throwing half his pitches outside the strike zone and giving up a run per inning.

In truth, though, no matter how occasionally great he may be, if he were going to start doing the things that would turn him into a great pitcher, he’d almost certainly have done so by now. To think that he’s going to change into something new just because hard-throwing lefties such as Sandy Koufax and Randy Johnson did relatively deep into their careers is to succumb to a blatant and obvious fallacy. On the evidence of 156 major league starts, Oliver Perez is a 4.51 ERA pitcher, a hair worse than average for a starter, capable of the odd flash of greatness, and not especially durable. It’s not nothing, but the odds are that if Perez, a pending free agent, leaves after this season, he won’t do much to make anyone miss him.

tmarchman@nysun.com