5.13.4+
5.14.0+

##</code><code>##

perl -E"$x=x; $x x= 100e6; <>; $r = \substr $x, 50e6; <>; ord $$r; <>"

AP 5.12.4 32-bit           99M                       148M         148M
AP 5.14.0 32-bit           99M                        99M         148M
AP 5.14.2 32-bit           99M                        99M         148M

##</code><code>##

>\progs\perl5142-ap1402\bin\perl -MDevel::Peek -E"$x=chr(0); $x x= 100; Dump substr $x, 50;"
SV = PVLV(0x4de51c) at 0x497b04
  REFCNT = 1
  FLAGS = (TEMP,GMG,SMG)
  IV = 0
  NV = 0
  PV = 0                                   <----- No buffer
  MAGIC = 0x4cc694
    MG_VIRTUAL = &PL_vtbl_substr
    MG_TYPE = PERL_MAGIC_substr(x)
  TYPE = x
  TARGOFF = 50
  TARGLEN = 50
  TARG = 0x4a932c
  SV = PV(0x25603c) at 0x4a932c
    REFCNT = 2
    FLAGS = (POK,pPOK)
    PV = 0x4b36cc "\0\0\0...\0\0\0"\0
    CUR = 100
    LEN = 104

##</code><code>##

>\progs\perl5124-ap1205\bin\perl -MDevel::Peek -E"$x=chr(0); $x x= 100; Dump substr $x, 50;"
SV = PVLV(0x319e4c) at 0x3bf54
  REFCNT = 1
  FLAGS = (PADMY,GMG,SMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x2f42dc "\0\0\0...\0\0\0"\0        <-----
  CUR = 50
  LEN = 52
  MAGIC = 0x327f94
    MG_VIRTUAL = &PL_vtbl_substr
    MG_TYPE = PERL_MAGIC_substr(x)
  TYPE = x
  TARGOFF = 50
  TARGLEN = 50
  TARG = 0x2f875c
  SV = PV(0x36034) at 0x2f875c
    REFCNT = 2
    FLAGS = (POK,pPOK)
    PV = 0x330f6c "\0\0\0...\0\0\0"\0
    CUR = 100
    LEN = 104