Student work in Perusall is graded automatically; to see the grades assigned, click the Gradebook button at the top of your course home page.
Perusall also provides you with an automatically-generated Confusion Report that summarizes general areas of questions/confusion. Click one of the assignments in the Assignments panel on your course home page to highlight it, and then click "Confusion Report" from the toolbar.
You can also review student work in more granular detail, in two ways:
Open up the assignment and then you can see all of the highlights and conversations as students see them. If you want to read through all of the threads, one easy way to do it is to click All Conversations on the right (the two bubble icons) and then you can click one to read, then click the icon at the top to go to the next conversation or go back to the list, etc.
If you would rather review the comments in spreadsheet format, go to your course home page, click to highlight the assignment in the right Assignments panel, and then select "All comments" from the "..." dropdown button in the toolbar.