substr($_, 3, 2) = 'foo';

##</code><code>##

substr($_, 3, 2, 'foo');

##</code><code>##

my $ref = \substr($_, 3, 2);
$$ref = 'foo';

##</code><code>##

>perl -MDevel::Peek -e"$_ = 'abcdef'; my $ref = \substr($_, 3, 2); Dump($$ref);"
SV = PVLV(0x4d3d24) at 0x2cb29c      <--- PVLV
  REFCNT = 1
  FLAGS = (TEMP,GMG,SMG)             <--- "get" magic and "set" magic
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x49f09c
    MG_VIRTUAL = &PL_vtbl_substr     <--- Function pointers for magic
    MG_TYPE = PERL_MAGIC_substr(x)   <--- x = substr magic
  TYPE = x                           <--- x = substr magic
  TARGOFF = 3           <--- For "x", start offset of substring
  TARGLEN = 2           <--- For "x", length of substring
  TARG = 0x4a8414       <--- For "x", addr of associated scalar ($_)
  FLAGS = 0
  SV = PV(0x2c8a6c) at 0x4a8414  <--- Dump of associated scalar ($_)
    REFCNT = 2
    FLAGS = (POK,pPOK)
    PV = 0x2cab94 "abcdef"\0
    CUR = 6
    LEN = 12