Thanks for reporting your findings, I too am a great believer in empirical evidence, even if only as a prelude to verification via other routes. Deriving this understanding from the Perl sources would require considerably more familiarity than I currently have, or am likely to expend the time to aquire. Whilst I love Perl as a language, I absolutely hate having to delve into the source, it's like reading a completely different language rather than C.

<personal grouse>Effective as it is, why oh why isn't it at least indented properly. In this day an age when I routinely use a CLI window that has 160 characters of width and often push this to 200 when the need arises, why does everything have to be squashed up into the left-hand 50 chars? </personal grouse>

I'm not sure that I'll be making any use of the information though. Once you see what is involved in an LVALUE

 perl> use Devel::Peek
 perl> $r = \substr('the quick brown fox', 10, 5)
 perl> print $r
LVALUE(0x1bd2a70)
 perl> print $$r
brown
 perl> Dump($r)
SV = RV(0x1bd1cc0) at 0x1bd29f8
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x1bd2a70
  SV = PVLV(0x1bc3388) at 0x1bd2a70
    REFCNT = 1
    FLAGS = (PADMY,GMG,SMG,pPOK)
    IV = 0
    NV = 0
    PV = 0x1bc37a8 "brown"\0
    CUR = 5
    LEN = 6
    MAGIC = 0x1bc3338
      MG_VIRTUAL = &PL_vtbl_substr
      MG_TYPE = 'x'
    TYPE = x
    TARGOFF = 10
    TARGLEN = 5
    TARG = 0x1bdf074
  SV = PV(0x1bcd490) at 0x1bdf074
    REFCNT = 1
    FLAGS = (PADBUSY,POK,pPOK)
    PV = 0x1bc19f0 "the quick brown fox"\0
    CUR = 19
    LEN = 20
[download]

you realise that LVALUE refs are far from the 'cheap' utility that they appeared (to me at least) to be. That combined with their undocumented nature probably outweights any advantages that they might have provided.