I agree with both what Kenta Murata and what Jeremy Evans wrote (to explain,
I have no personal preference; I think both .p() would be useful anyway and
I think this was the spirit of the original suggestion; I love p and pp).
Martin suggested warn_p so I think this addresses more Kenta Murata, but I
think, personally, the net benefit of warn_p may be significantly smaller
than the "puts" versus "p" situation, where I think the gain is quite
significant here, if you write a lot of code that makes use of p for
output. (This is a bit similar to the proposal to do "require 'pp'" by
default, which was a good change IMO).
Anyway, my personal opinion is +1 to the idea behind the proposal - I have
no real preference on it being on STDERR or $stderr or both; I think it may
be useful on its own, no matter where the method ultimately resides.