If I can find it, I have an old post with images that shows the white ring of unprinted vinyl around each perforation, like Fred mentioned... but it also shows a close=up of a second brand of perf that did not leave that unprinted ring. The difference was a dimpling of the material around each perforation on the "bad" stuff... & a good flat surface on the stuff that worked. I have to log off & drive to work, so may not remember to do this.
Email me Brian, if that product information interests you & I'll try to look it up
(I may even have a roll to sell... now that I have an inkjet)