Yes - when you descend to that level they become very similar, indeed :-)

At a high level:

	validate $x	defined &$x	ref $x && defined &$x
code ref	returns result	returns result	returns result
sub name	dies	returns result	returns false
other refs	dies	dies	dies
undefined	?	stringified	returns false

I couldn't even get correct results on perl-5.6.2 with CvROOT and CvXSUB

That's odd, because defined and sub calls use CvROOT(cv) || CvXSUB(cv) in 5.6.2 as well.