For all of you (considering) using %_, this just in from p5p by TheDamian:
in reply to Perl 5 -> 6 do's and don'ts?
From a Perl 6 perspective, it seems likely that C<%_> will be the name commonly used for the "slurpy hash" of a subroutine. Just as C<@_> will often be the name used for the "slurpy array". See Exegesis 6 for more details.
Indeed, when it comes to object constructors (all of which implicitly have a slurpy hash), C<%_> might well be the automatically provided name for that hash. See Exegesis 12 for more details. ;-)
Hence, making C<%_> mean something different in core Perl 5 might possibly be "forwards incompatible"