Once you press Enter/return to submit a comment, it is saved and will automatically count towards your score for the corresponding assignment. There is no need to explicitly submit your assignment when you are done!
In fact, research shows that you will learn more if you come back to the assignment multiple times to continue to participate in discussions (rather than completing it all in one sitting), so you should always come back to the assignment later on to ask more questions or add additional comments.
In order for Perusall to gather accurate data, remember to always complete your assignments through the Assignments Portal (for a standalone Perusall.com course) & through Assignment-specific links (for Learning Management System courses, like Blackboard, Sakai, Canvas, Moodle, Desire2Learn/Brightspace, etc).