use Devel::Peek;

my $x = "foo";

Dump $x;
vec($x, 0, 2) = 3;
Dump $x;

##</code><code>##

SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "foo"\0
  CUR = 3
  LEN = 8
SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "goo"\0
  CUR = 3
  LEN = 8