Thanks for the explanation. I guess a more thorough reading of the docs should have sorted this out for me.
However, do you think this is the way it should work? After all you don't expect your variables to suddenly change as you go through a function. I know $_ is a global, but I imagine a lot of people write code like the stuff I was maintaining where there is a call to a sub in the middle of using $_. And then maybe that calls a sub and that calls another one, etc. Then someone comes along one day and puts while(<INPUT>) in one of those subs, all careful with use strict and warnings, but suddenly some bit of code miles away just breaks!
Anyway, I know I'm getting a bit hot under the collar over this, perhaps because I spent about an hour not knowing why my change had broken the whole program. But shouldn't there be some kind of health warning at least over the use of while(<>) since it must be one of the first idioms beginners learn.
I know I'm going to write
from now on!
update No I'm not I'm going to follow jeffa's example! Doh! I might go through and put local in the old "code"/plate of spaghetti that I'm supposed to be maintaining.