Certainly while( <> ) { ... } works as I expected it to. While talking it over with other helpful monks via the Chatterbox, I found that the individual functions will say if they use $_ in their descriptions.

That being said, is there a good reason why shift (for example) doesn't assign a value to $_ unless another variable is explicitly used?

Because, for one thing, it's useful to call shift in void context to remove an array element with no other side-effects. Clobbering $_ in these cases would make it less useful.