"for their own mutual safety and welfare" and "binding the said Colonies to assist one another against all Force offered to or attacks made upon them or any of them"

Now even though it is a perpetual uninon, it is ONLY for the saftety and welfare of the Group. So to assume the Union was perpetual for any other purpose would be wrong. It was not a Union of the States to form a Government to rule them, it was only for defense and to conduct commerce amongst each other. So once again, Secession was legal.