I'm sorry I confused the issue. My question wasn't "why doesn't while assign a value to $_" but "why doesn't shift?"

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?