Seems there isn’t a clear definitive answer via the Congress nor the Supreme Court regarding the issue of in-state tutuion.
Here is a short excerpt on the topic and I’m sure there is more out there but I’m not going on a hunt.
http://www.ncsl.org/default.aspx?tabid=13100
If there’s not a clear answer, meaning that it hasn’t been clarified in the courts, then I’d argue that means you go by the letter of the law, which to my mind is pretty clear. Seems to me Perry and his legislature are trying to do an end run around federal law. I have HAD IT with pols treating the Constitution like toilet paper. And I don’t give a tinker’s dam which party they belong to.