That only fired the Pumas and Serra scored their opening try, after good work in midfield from Martin Gaitan, before opposite wing Carballo bagged his first touch down.
The England star darted to the right of a ruck 10m inside the Castres half, stepped past Masoe and wing Rafael Carballo before racing clear to touch down under the posts.
But they were unable to close the gap further in the closing 10 minutes when a Serra up-and-under caused confusion in the home ranks from which Carballo profited and Agulla rounded off the scoring.