It looks like it was an agreement for self protection. It kept others from dropping out or from not offering assistance if one of them were under attack. Like the Three Musketeers so to speak.
Had the congress believed it was really 'perpetual', would they not have been fighting in the 1860's to save the Union and NOT free the slaves? We've discussed this before but I don't remember this part being in the discussion. If South Carolina removed itself from the 'Perpetual' union, should the others have fired on it saying they started it or taken the stand that they had an agreement they were abiding by and not fought back? I know, tempers and all, but this wording takes some of the bite from the 'other' argument as to what it was all about. At least to my uneducated in this area way of thinking.

Pam