What clause in the Constitution attests to it's perpetuity?
If the framers contemplated states coming and going from the union, they would have provided structure for it.
They did - read the Tenth Amendmenment.
The confederacy was formed to protect and expand slavery, which is tyranny in its purest form.
You wield a wide brush - I disagree with your premise. The several states - from their Declaration of Independence from Britain, into the Articles of Confederation, and into the Constitution all supported slavery, as did much of the known world (still practiced today in Africa).
What clause in the Constitution attests to it's perpetuity?
Every clause, and every word.
"Whoever considers, in a combined and comprehensive view, the general texture of the constitution, wil be satisfied that the people of the United States intended to form themselves into a nation for national purposes. They instituted, for such purposes, a national government complete in all its parts, with powers legislative, executive and judiiciary, ad in all those powers extending over the whole nation. "
Justice James Wilson, opinion in Chisholm v. Georgia, 1793.
No one challenged what Wilson said.
It was for a later generation to spout the treason of independent states, co-equal in a compact.
Walt