The last time I looked, which was about 14 months ago, Florida had no law(s) on window perf...just laws on window tint.
No case law that I am aware of which would include perf in the tint category.
I can't speak for any other state but in Florida you can completely opaque rear
windows or not have any at all (think some types of cargo vans) as long as you have adequate side mirrors.
Rear passenger side glass is treated the same way.
The only thing that does matter is the front two side
windows which must remain unobstructed (have to see and use the mirrors) plus you can only semi-opaque/tint/obstruct a small portion of the windshield in the upper area.
All that surprised me as violation of the tint rules had severe consequences yet you didn't even have to have glass in that area. Mis-tint and pay a fine, paint over or remove the window and you are good to go. Go figure!
Commercial passenger vehicles such as limos have slightly different rules for tint than std. personal vehicles.