I believe that it is close to the notion of "auto-vivification." An object is created within the interpreter to reflect the fact that you have referred-to this thing, and you see the (dummy) object that it made, but it has no actual definition and therefore no content. CORE::GLOBAL like all of CORE is a pseudo-namespace that exists because a namespace is needed not because it is actually real in the sense of other namespaces which appear in source code.
Yes auto-vivification is close, the analogy becomes clearer since there is a hash like syntax to access the CODE slot of a typeglob. (like *CORE::sleep{CODE} )
anyway the important question is to know if a real coderef exists.
That's why you can check this with exists &Func; (sic)