But this undercuts your argument. I agree that the Articles stated that it was a perpetual union, but that just demonstrates that the framers, if they were so inclined, easily could have so specified in the Constitution. But they did not. The framers were all bright, intelligent people. If they had meant for the Union to be perpetual, why did they not specify that in the Constitution as they had the Articles?
Because a basic rule of construction is to assume that the inclusion or absence of clauses is deliberate, why do you think that the framers specified a perpetual union under the Articles and the deliberately chose not to specify that under the Constitution? This certainly points to the conclusion that our Constitutional union is not perpetual, but is rather one that can be exited (though not entered) at the will of the states.
Let me quote Chief Justice Chase on that question: "The Union of the States never was a purely artificial and arbitrary relation. It began among the Colonies, and grew out of common origin, mutual sympathies, kindred principles, similar interests, and geographical relations. It was confirmed and strengthened by the necessities of war, and received definite form, and character, and sanction from the Articles of Confederation. By these the Union was solemnly declared to 'be perpetual.' And when these Articles were found to be inadequate to the exigencies of the country, the Constitution was ordained 'to form a more perfect Union.' It is difficult to convey the idea of indissoluble unity more clearly than by these words. What can be indissoluble if a perpetual Union, made more perfect, is not?"
But that is neither here nor there, because I'm not arguing that the Union is perpetual or that states cannot leave the Union and neither was Chief Justice Chase. The question is the method which states leave. Can they just walk out, without discussion, regardless of how much harm their actions cause the remaining states? Or should withdrawl be negotiated by both sides to ensure that the interests of all are protected? I cannot believe that given the other restrictions the Constitution places on the states and the powers granted to Congress to literally create states in the first place that the Founders meant for secession to be unilateral.