LVALUE is used internally for the 'magic' used with substr, vec, pos, keys, etc. when you use them as lvalues in an assignment. It holds (in addition to typical scalar data) a pointer to the Sv* that it can modify and what part to modify.
PerlGuts Illustrated has a lot of information on how Sv*'s are layed out. In reguards to your original question is shows that SvRV (a struture that holds only a reference to another Sv*) is smaller than SvPV, SvPVIV, and SvPVNV, which are the 3 that I'm guessing are considered 'SCALAR' by ref. So 'REF' might indicate that the underlying structure is to small to hold a full scalar value, and would need to be upgraded to a SvPV of better to hold a non reference value. I don't know that any of that information would be of any use to Perl code, and perl handles all that on the C level so 'REF' may be only informative, or entirely redundant, depending on your viewpoint. (but as others have noted ref is less than totaly useful it self as it stands now)
Disclaimer: I've only recently started looking at the internals of perl, I could be totaly off mark on any of this.

In reply to Re^5: ref == "REF" by Ven'Tatsu
in thread ref == "REF" by gaal

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