use Devel::Peek;
{ my $subRef = \substr $string, 0;
  Dump($$subRef);
}
{ my $subRef = \(''.substr $string, 0);
  Dump($$subRef);
}

##</code><code>##

SV = PVLV(0x1834fdc) at 0x1831820
  REFCNT = 2
  FLAGS = (PADMY,GMG,SMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x18208ec ""\0
  CUR = 0
  LEN = 4
  MAGIC = 0x182443c
    MG_VIRTUAL = &PL_vtbl_substr
    MG_TYPE = PERL_MAGIC_substr(x)
  TYPE = x
  TARGOFF = 0
  TARGLEN = 0
  TARG = 0x236dc8
  SV = PV(0x238e44) at 0x236dc8
    REFCNT = 2
    FLAGS = (POK,pPOK)
    PV = 0x182eca4 ""\0
    CUR = 0
    LEN = 4
SV = PV(0x238e80) at 0x1831808
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x183646c ""\0
  CUR = 0
  LEN = 4

##</code><code>##

my $subRef = \(''.substr $string, 0);

##</code><code>##

0.0120670795440674
0.0105710029602051
0.010854959487915

##</code><code>##

my $subRef = \scalar substr $string, 0;