Perusall lets you choose when to release grades to the students–automatically once the due date is passed, or manually. That said, the automatic function can not result in what you expect because if you extend the due date for even one student, it can delay the release until that extension due date. So, even if you set the release to be automatic, you may find yourself wanting to release grades manually.
Step-by-step guide
In Perusall, click on the course.
Click on the "Gradebook"
Scroll to the bottom of your grades and click on the yellow "Release to Students" box at the bottom of the appropriate column.
Related articles