You can certainly grow your perennials on a raised mound. In the landscaping world, this is commonly called a berm.
There's only one basic rule --- and that's to make the berm 4 or so times wider than it is tall. That will help keep it stable so it doesn't erode too quickly while your perennials are getting established.
Also: It will tend to look more natural if you give your berm an irregular shape instead of making it a perfect circle.
---Justin, Senior Garden Editor, BHG.com