It sounds like your cat has pica, or an appetite for non-nutritive/edible substances. Many cats have this, particularly for plastic, my cat Molly included. She doesn't go as far as to tear off chunks and actually swallow it, but when she's hungry, she always licks plastic.
As mentioned above, this can become a serious problem if it causes an intestinal blockage. Even if it never gets that bad, it's still not good for her. Plastic has all sorts of chemicals in it that mimic our bodies hormones, and the same goes for our cats. This could lead to behavioral problems and other health issues like cancer later on.
Is she only eating the plastic from the litter box liners? If so, get rid of the litter box liners. I don't know what kind of litter box you use, but I'm sure it works just fine without the plastic liners. Also make sure that you don't leave any plastic bags lying around or any other types of thin plastic materials that she can easily chew and swallow.
I hope that helps!