Is there any document that prevents states from seceding the union?

Answer 1

No. The Constitution is silent on the issue.

But right up until 1861, everybody knew that OF COURSE you could leave the Union, if your state wanted to. It had taken a voluntary act by the states to join the Union, so why would a voluntary act by any state not be enough to end the association?

In the years between the Revolution and the Civil War secession was seriously considered several times, at least twice by the New England states and once by the new states of Ohio, Kentucky and Tennessee, in the 1790s. The new states were concerned that the Federal government did not seem to care that the only outlet for their trade was down the Mississippi, which was in Spanish hands, and the Spanish would not allow their goods to get out through New Orleans. The New England states were also dissatisfied in the 1790s and tried to raise enthusiasm for separating, but they could not get New York to go with them, so the idea died. Then, during the War of 1812, the idea came back with a vengeance. The New England states had swiftly resumed trade with Great Britain before the echo of the last shot of the Revolution had died out, and were bitterly opposed to the War of 1812. New Englanders grew rich carrying on a treasonable trade with the enemy during the War, supplying the armies of Britain with food and supplies, without which Britain could not have maintained its forces in North America. The New England states called a secession convention at Hartford, Connecticut in 1815 (The Hartford Convention) to secede from the Union. By the time the convention met, the war had ended, so the idea was dropped.

So the US did not become "perpetual" and "indivisible" until the south tried to secede.