The Capitals scored three times in a 6:21 span early in the second to erase a 2-0 deficit. Washington, which also had two goals disallowed, had been held to 11 goals during its 0-6-2 skid.
"We felt like we won a championship," Perreault said. "We needed that win so much for our confidence and just for the team. That was a huge win for us."
The Capitals' players poured off the bench at the end of the game and gathered in a mob around goalie Michal Neuvirth, knocking the net up against the boards.
"In the sense that we'll be able to sleep tonight I think it was a big win," coach Bruce Boudreau said. "And as a coach, just looking at the players' faces and seeing them happy instead of grinding and everything it's a huge win, but in the end it's one win out of nine. It's a relief for us but it's not like we're out of the woods."
Ryan Shannon and Chris Kelly scored in the first for Ottawa, which hadn't lost through regulation in its previous four games (2-0-2).
"We got the start we wanted, they got some good breaks early in the second and that's five minutes we would like to have back, but that's the way it goes," Senators captain Daniel Alfredsson said.
Alfredsson acknowledged that Washington's victory celebration was a bit unusual for a midseason game, referencing the HBO camera crews on hand to film the Capitals for the cable network's reality series.
"I'm sure they know they're on TV and they've got a show going on so that's I'm sure a good reason," Alfredsson said.
Perreault, who was sick and didn't play Saturday night in a loss in Boston, gave Washington its first lead on a power play 6:55 into the second for his second goal of the middle period.
He scored his first goal of the game 34 seconds into the second to draw the Capitals to 2-1. Fehr, who assisted on Perreault's second goal, tied it at 2 with his fifth goal at 1:20 after having another waved off when time expired at the end of the first.
"We knew we had to win that game," Perreault said. "After the first period we all regrouped and said, 'OK guys, this has got to be our best period of the year.' And I think that's what we did."
Neuvirth stopped 24 shots for Washington, which also had an apparent goal by Alex Ovechkin disallowed later in the second. Brian Elliott made 31 saves for Ottawa.
"When you get two points you're just happy like a kid," said Ovechkin, who got his team-leading 26th assist and 38th point on Fehr's goal. "We just want to win so bad."
The Capitals' captain saw his goal drought extended to six games, though, when a video review wiped out his tally 9:14 into the second which would have put Washington up by two.
Elliott stuck out his left pad to stop Ovechkin's drive with just under 4 minutes remaining. Ovechkin has scored just once in his last seven games, and twice in 17.
Washington struck for two quick goals 46 seconds apart early in the second, taking advantage of a pair of Ottawa turnovers to tie it at 2.
Former Capitals defenseman Sergei Gonchar turned over the puck at the Capitals' blue line and Brooks Laich took off on a 3-on-1. Perreault tapped home Mike Knuble's rebound for his third goal.
Ovechkin intercepted veteran Ottawa defenseman Chris Phillips' pass along the boards behind the net moments later and fed the puck to Fehr, who fired it past Elliott to tie it at 2.
Perreault gave Washington the lead on a power play at 6:55 with his fourth goal, his second of the period.
Ovechkin appeared to have scored when a rebound of Laich's tight-angle shot from the right corner went in off his left leg. Referee Don VanMassenhoven signaled a goal but his on-ice ruling was overturned by a video review, which showed Ovechkin turned his left skate out as he drove toward the net.
"I kicked it," Ovechkin said. "If it was in, it was in, I'll take it. Why not?"
Fehr just missed scoring at the end of the first period. His shot got past Elliott but the tally was waved off immediately by VanMassenhoven. A video review confirmed that time expired before the puck went in.
Shannon put a wrist shot past Neuvirth for his fourth goal 7:11 in.
Kelly made it 2-0 with his eighth goal at 13:55, snapping a shot past Neuvirth from the edge of the crease.
Well before VanMassenhoven announced that Ovechkin's goal had been disallowed, the other referee, Eric Furlatt, skated over to move the puck from the center-ice faceoff dot to the one outside the Washington blue line in front of the Capitals' bench. ... Capitals LW Alexander Semin missed his third game in a row because of a lower-body injury. Semin leads Washington with 18 goals in 32 games, though he had not scored in seven games prior to his injury.
Copyright by STATS LLC and The Associated Press
Arena: Canadian Tire Centre Location: Ottawa, Ontario
Referees: Eric Furlatt, Eric Furlatt Linesmen: Brian Murphy, Mark Shewchyk
Attendance: 19,516 (105.5% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Ryan Shannon (4)
Assist: Jesse Winchester
Chris Kelly (8)
Assist: Chris Neil
Mathieu Perreault (3)
Assists: Mike Knuble, Brooks Laich
Eric Fehr (5)
Assist: Alex Ovechkin
Mathieu Perreault (4)
(Power Play) Assist: Eric Fehr