As for the additional memory, when you Devel::Peek::Dump() the reference (with a shorter string), you can see that a PV has been created after having accessed the string (via "magic").  My first guess would be this is some caching behavior to increase performance with repeated accesses.

use Devel::Peek;

$v = chr(0); $v x= 50;
$r = \substr $v, 25;
Dump $r;

vec( $v, 25, 8 ) = 0b10101010;
print vec( $$r, $_, 1 ) for 0..7;
Dump $r;

__END__
SV = IV(0x7991e0) at 0x7991f0
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x771998
  SV = PVLV(0x7a1ce0) at 0x771998
    REFCNT = 1
    FLAGS = (GMG,SMG)
    IV = 0
    NV = 0
    PV = 0
    MAGIC = 0x7914f0
      MG_VIRTUAL = &PL_vtbl_substr
      MG_TYPE = PERL_MAGIC_substr(x)
    TYPE = x
    TARGOFF = 25
    TARGLEN = 25
    TARG = 0x7991c0
    SV = PV(0x76fc20) at 0x7991c0
      REFCNT = 2
      FLAGS = (POK,pPOK)
      PV = 0x7a4f50 "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\
+0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"\0
      CUR = 50
      LEN = 56
0
1
0
1
0
1
0
1
SV = IV(0x7991e0) at 0x7991f0
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x771998
  SV = PVLV(0x7a1ce0) at 0x771998
    REFCNT = 1
    FLAGS = (GMG,SMG,pPOK)
    IV = 0
    NV = 0
    PV = 0x7917e0 "\252\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\
+0"\0         # <---
    CUR = 25
    LEN = 32
    MAGIC = 0x7914f0
      MG_VIRTUAL = &PL_vtbl_substr
      MG_TYPE = PERL_MAGIC_substr(x)
    TYPE = x
    TARGOFF = 25
    TARGLEN = 25
    TARG = 0x7991c0
    SV = PV(0x76fc20) at 0x7991c0
      REFCNT = 2
      FLAGS = (POK,pPOK)
      PV = 0x7a4f50 "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\
+0\252\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"\0
      CUR = 50
      LEN = 56
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