How about we start with the construction of Congress? In the AOC Congress was designed as unicameral; in the US Constitution it became bicameral with both a House of Representatives and a senate.
The Congress, as described in the AOC, does not exist in the Constitution.
The concept of perpetual union must be seen exactly that way. It was discussed in the AOC -- but not in the Constitution. Therefore, it's not true today. If the Founders wanted to transfer the good concept of perpetual union from the AOC into the Constitution, they would have done so. They did not. Therefore there are no grounds to say that our union is perpetual.