the array is evaluated in a list context.

Huh, it sounds a bit like a tautology, doesn't it? (Even though an array may be evaluated in a scalar context and return in this case the number of elements.)

I would rather say that the array is imposing a list context and that the chomp operator is used in a list context in this case. And, in such a case, it chomps all the elements of the list (and returns the total number of characters removed)

Note that if used on a hash, it chomps the hash's values, but not its keys.