5.6.x - 5.8.x: sort CORE::keys() -> undefined subroutine in sort

(I apologize ahead of time if this is a FAQ or FSB (Frequently Submitted
Bug).  I ran this through perlbug, but (1) I don't know how often
p5p-types go through those bugs, and (2) I left out the word "undefined"
in the subject anyway.)

I get the following on all platforms for all versions perl I've tried
5.6.x or later:

 $ perl -we '%a=(foo=>"a"); print((sort CORE::keys(%a)),"\n")'
 Undefined subroutine in sort at -e line 1.

Whereas,

 $ perl -we '%a=(foo=>"a"); print((sort keys(%a)),"\n")'
 foo

james

• 5.6.x - 5.8.x: sort CORE::keys() -> undefined subroutine in sort by James Jurach