perldoc perllexwarn
has a great tree representation of the warnings heirarchy so if you're ever in a situation where you want to turn off certain type of warning, you can see were it fits in the "tree" and only turn off the bit you need and not disable all warnings.
I'm aware of this, but I'm on a (silly, I know) quest for ways to avoid these annoying messages without setting up a block solely for this purpose.
BTW, when I want to find out the category of warnings to turn off I trigger the warning under diagnostics, whose output lists the categories of the warnings emitted. I don't think I've ever had reason to avail myself of the hierarchical organization of warnings; I always turn off the most specific level corresponding to a particular warning.