Arrays are parameters. Everything described in the section on
parameters applies to arrays (and associative arrays / hashes) as
well. The semantics may differ slightly -- sometimes every array
element is affected individually, sometimes the whole array is
examined. For $+name (which is shorthand for ${+name}), it's the
whole array.
But the quoted snippet doesn't use bare $+name; it uses $+name[index], which does return the appropriate result for that specific element of the array.
Worth mentioning that $+name is 1 even for an empty array, where $+name[1] in that case will be 0.