The following are only part of the documents which forbid secession from the Perpetual Union. Thee are other provisions which implicitly restrict the ability to secede without the consent of Congress, the States, and the seceding State.
Articles of Confederation and perpetual Union
XIII.
Every State shall abide by the determination of the United States in Congress assembled, on all questions which by this confederation are submitted to them. And the Articles of this Confederation shall be inviolably observed by every State, and the Union shall be perpetual; nor shall any alteration at any time hereafter be made in any of them; unless such alteration be agreed to in a Congress of the United States, and be afterwards confirmed by the legislatures of every State.
And Whereas it hath pleased the Great Governor of the World to incline the hearts of the legislatures we respectively represent in Congress, to approve of, and to authorize us to ratify the said Articles of Confederation and perpetual Union. Know Ye that we the undersigned delegates, by virtue of the power and authority to us given for that purpose, do by these presents, in the name and in behalf of our respective constituents, fully and entirely ratify and confirm each and every of the said Articles of Confederation and perpetual Union, and all and singular the matters and things therein contained: And we do further solemnly plight and engage the faith of our respective constituents, that they shall abide by the determinations of the United States in Congress assembled, on all questions, which by the said Confederation are submitted to them. And that the Articles thereof shall be inviolably observed by the States we respectively represent, and that the Union shall be perpetual.
THE CONSTITUTION OF THE UNITED STATES OF AMERICA
Article. IV.
Section. 3. New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, ithout the Consent of the Legislatures of the States concerned as well as of the Congress.
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Section. 4. The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
Article. VI.
All Debts contracted and Engagements entered into, before the Adoption of this Constitution, shall be as valid against the United States under this Constitution, as under the Confederation.
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any state to the Contrary notwithstanding.
A couple of observations...
Most, if not all, of these early documents referred to the united States of America. Small ‘u’. United was a condition, not part of a title. IOW, the colonies were a collection of sovereign States, united for common defense and to ensure that treaties with foreign nations, or, presumably, Indian nations, would include all the states, disallowing any of them from negotiating their own treaties. Surely the condition of unity could be changed by the individual state, or by the rest of that union against an individual state, if that unity was no longer in their best interest.
Perpetual union? Nothing is perpetual. Even the Third Reich was only guaranteed to last a thousand years and that guarantee turned out to be no good. Liberal bulls#it sometimes seems to be perpetual but it will end sooner or later, later if I have anything to say about it.
The language you quoted from the U.S. Constitution says nothing at all about the Union being perpetual, or about whether and how secession could occur.