Ovechkin took three shots and delivered several checks. With 3:03 left in the game, Ovechkin got into a shoving match after being run into the boards headfirst by Gregory Campbell. Ovechkin retaliated by slashing Campbell with a one-handed stick swing, and both players earned minor penalties.
The Capitals are unbeaten in three exhibition games.
Perreault and Reich traded first-period goals. With Ovechkin assisting, Perreault connected on a wrist shot from the right circle 8:40 into the game. But Reich beat Michal Neuvirth from the left wing on a counterattack at 14:13.
With 46 seconds left in the second period and Reich serving a minor penalty, Semin gave the Capitals a 2-1 lead. He converted Ovechkin's feed by ripping a hard shot from the left circle past Nolan Schaeffer.
Eakin's goal made it 3-1 with 14:33 remaining in the third. Eakin used defenseman Adam McQuaid as a screen and scored from between the circles.
Marchand scored from the right wing with 9:04 left to get Boston within 3-2.
Copyright by STATS LLC and The Associated Press
Arena: Verizon Center Location: Washington, District of Columbia
Referees: Dave Jackson, Greg Kimmerly Linesmen: Scott Cherrey, Tim Nowak
Attendance: 17,335 (94.2% full)
Mathieu Perreault 1 Assist: Alex Ovechkin, Mike Knuble
Jeremy Reich 1 Assist: Matt Bartkowski
Alexander Semin 1 (power play) Assist: Alex Ovechkin, Marcus Johansson
Cody Eakin 2 Assist: Matt Bradley, Tom Poti
Brad Marchand 1 Assist: Milan Lucic, Matt Bartkowski