Mark DeRosa was also reinstated from the 15-day disabled list after he missed the last 25 games with an abdominal injury. The 37-year-old utility man is batting .152 (10-for-66) with six RBIs in 34 games.
Lannan has been an expected September callup for much of the season, especially with Stephen Strasburg's shutdown looming later this month. He made two starts with the Nats earlier this season -- both in doubleheaders -- and went 2-0 with a 3.46 ERA, seven strikeouts and seven walks in 13 innings.
Manager Davey Johnson said he doesn't have a specific plan in mind for Lannan yet, though he will eventually slide into the rotation. Should the Nats reach the postseason, the team could go with a four-man rotation. That, though, is a decision Johnson won't be making quite yet.
"I think he's going to throw a side [session on Sunday]," Johnson said. "I wouldn't pitch him for two more days anyway, because that would be his regular time to start. At this length of year, he had a good game last time out. I'm not even concerned about his workload for the next eight days."
At Triple-A Syracuse this season, Lannan went 9-11 with a 4.30 ERA. The 27-year-old left-hander also blossomed in his final two starts with the Chiefs, throwing two straight shutouts to end his season in the Minors. He was named the International League Pitcher of the Week last week.
Johnson said Lannan could soon pitch out of the bullpen before joining the rotation. On Thursday, Johnson said Strasburg has "a few more starts" this season, either two or three.
Perez, meanwhile, joins the Nats after spending the season with the Gulf Coast Nationals, Double-A Harrisburg and Triple-A Syracuse, batting .314 with 42 RBIs overall. Ranked No. 5 among the Nationals' top prospects, the 22-year-old outfielder will be used primarily as a pinch-runner. Johnson joked Perez will replace Edwin Jackson -- not as a pitcher, but as a late-game, last-ditch pinch-runner.
"I'm not big on trying to steal a base," Johnson said. "I don't care how good you are, you can still get thrown out. It's nice to have that option. With Edwin, I've used him -- I don't know how many times -- but there's been some other times I've used him and he didn't even have his shoes on.
"A few times, I can run Eury out and give him an at-bat, lead off an inning or pinch-hit, play center field. Stuff like that."
Leon rejoins the Nats after batting .250 in 10 games with the team earlier in the season, mainly in July. Leon batted .322 with Class A Auburn, Harrisburg and Syracuse this season.
The 23-year-old will be used as the third catcher behind Kurt Suzuki and Jesus Flores, as well as a pinch-hitter.
Overall, Johnson said the Nationals didn't elect to call up many prospects because of the team's success this season. The Nats were the second team to reach 80 wins this season and entered Saturday with a 6 1/2-game lead over the Braves in the National League East.
"I've been in this situation before, it's not my first rodeo," Johnson said. "We have what we need here without any reinforcements. These other guys get some big league time, some big league experience.
"We're no longer developing, in case you haven't noticed."