I grew up in Kentucky and in those days (virtually) EVERYONE was a Democrat. You could be liberal or conservative but, if you wanted to be elected, you had to call yourself a Democrat.
I used to wonder why Kentucky held its election for Governor in May when other states held theirs in Nov. Actually it was the Democrat primary that was held in May, the one with all the TV and newspaper ads and all of the hoopla, i.e., the one that mattered. The actual Nov. election was a foregone conclusion as to the winner between the Democrat candidate and the Republican candidate. So if you wanted your vote to count, you had to be registered as a Democrat.
When I moved to Florida in 1980 it was same and I registered as a Democrat. But as I matured I found I could no longer live the lie and could no longer be associated with those people and that's when I switched my registration to Republican.
I suspect that Perry's situation was similar.
My parents moved to Florida and said the same thing. If you wanted your vote to actually count, you had to register as a Democrat to make your vote count.
They finally moved back to Republican before the 2000 election. They still voted mostly republican in the national election but the Republicans had almost no real local presence when they first got there.