The Articles of Confederation called the Union perpetual, if I recalled correctly, not the articles themselves (itself) per se.It uses the word "permanent", not "perpetual", though they mean basically the same thing ("perpetual" has a better ring to it, in my opinion). But this doesn't preclude the secession of a state. If Texas were to secede, the Confederate government would not cease to exist.
And, for what it's worth, the Articles of Confederation were said to be "perpetual" as well.