Just the Perian plug-in won't get you anywhere. Get the free MPEG Streamclip program to play and convert .flv files and many others (the Perian plug-in is still needed).
I've been experimenting with this as well. I have found imovie HD (free download) will allow you to import .flv files just fine. I don't understand why imovie (a newer version) doesn't have the same capability. Beyond my pay grade.