The 20-year-old striker, making his first start for the Baggies, was the only scorer in their 3-0 victory over the League Two outfit, who knocked out Championship side Brighton & Hove Albion at the first stage.
With Nicolas Anelka absent indefinitely for personal reasons, Clarke will be forced to look at his other forward options when West Brom return to Premier League action against Swansea City on Sunday, and has suggested that Berahino's performance has given him food for thought.
"He was excellent, really great," Clarke told reporters. "I'm really pleased for him because he was at Peterborough last year where he did ever so well in the Championship but then got a really nasty injury which kept him out for seven months.
"But he worked hard over the summer with the medical staff and got in great shape. I'm absolutely delighted for the boy because he's worked so hard for his chance and he took it with both hands.
"You saw his performance. In the first half he was the best player on the pitch. He tired a bit in the second half - as did a few others who haven't had much playing time recently. But he's given me a lot to think about and I'm really pleased he has."