A strong Saddlers XI, with seven changes, took on a young Rovers side in the last game of the group stage with Walsall dominating from the start.
Striker Josh Gordon picked up a hattrick just after half-time with a penalty before Wes McDonald, Liam Kinsella and Cameron Norman finished the rout.
It was an impressive display from a side low on confidence and Clarke is hoping to take this form into Saturday’s league game against Cambridge.
“We had to win a football game didn’t we,” he said.
“Obviously we know results haven’t been anywhere near good enough for this football club.
“You only do that by winning your next game.
“The lads put in a good performance, done the job and it’s nice to be in the next round.”
Clarke’s men pressed aggressively throughout the game and forced Rovers into mistakes with Gordon, Caolan Lavery, Rory Holden and McDonald all causing the back-line issues.
And Clarke expects his players to gain confidence from that for the upcoming fixtures.
“Forest Green are a very good team that are doing well in League Two.
“I know they made quite a few changes but for me the shape of the team was good without the ball.
“We pressed high, our transition was good when we did win it back and ruthless when we needed to be, which we haven’t shown at any point this season.
“Hopefully the boys gain a bit of confidence from that.
“I’m not getting carried away but we have to enjoy that and build confidence and try and take it into Saturday.”
Walsall have topped Southern Group D and are now through to the knockout stages of the competition.