Alex Grove and Simon Danielli grabbed the other tries before a late Youngdae Kim try got Japan on the scoreboard, and Barclay knows they need to up their game before they face the Pumas on June 12.
He said: "When we got the ball, we gave it away too cheaply. And, against a side like Argentina in two weeks' time, if we do that, we will be in a lot of bother.
"It was scrappy, but we came up against a really tenacious and aggressive Japanese team.
"At the break down, they were more aggressive than us and it made life really difficult for us.
"We got lessons learned as we know it is going to be like that in Argentina.
"So, a few issues to be looked at and resolved, but nothing that can't be fixed."