Mostly, it's about saying what you mean. @_[$a] connotes a slice. $_[$a] connotes a scalar. The warnings aren't to tell you that the program won't generate what it's supposed to (although there are situations where the array context a slice provides could cause curious behavior), they're to tell you you're programming in a semi-literate way.

For what it's worth, I don't say "golly". Nor do I hide behind Anonymous Monk posts when I want to be snotty.