$ perl -MDevel::Peek -e " my $a = q!1234!; Dump $a; "
SV = PV(0x3e6964) at 0x3e8b54
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x98f664 "1234"\0
  CUR = 4
  LEN = 8

$ perl -MDevel::Peek -e " my $a = q!1234!; substr$a,0,2,q!!; Dump $a; "
SV = PVIV(0x98005c) at 0x3e8c04
  REFCNT = 1
  FLAGS = (PADMY,POK,OOK,pPOK)
  IV = 2  (OFFSET)
  PV = 0x98f746 ( "12" . ) "34"\0
  CUR = 2
  LEN = 6

$ perl -MDevel::Peek -e " my $a = q!1234!; substr($a,0,2)=q!!; Dump $a; "
SV = PVIV(0x98005c) at 0x3e8c0c
  REFCNT = 2
  FLAGS = (PADMY,POK,OOK,pPOK)
  IV = 2  (OFFSET)
  PV = 0x98f746 ( "12" . ) "34"\0
  CUR = 2
  LEN = 6

$ perl -MDevel::Peek -e " my $a = q!1234!; substr($a,0,2)=q!!; substr$a,0,2,q!!; Dump $a; "
SV = PVIV(0x98005c) at 0x3e8c1c
  REFCNT = 2
  FLAGS = (PADMY,POK,OOK,pPOK)
  IV = 4  (OFFSET)
  PV = 0x98f778 ( "1234" . ) ""\0
  CUR = 0
  LEN = 4

$ perl -MDevel::Peek -e " my $a = q!1234!; substr$a,0,2,q!!; substr($a,0,2)=q!!; Dump $a; "
SV = PVIV(0x98005c) at 0x3e8c1c
  REFCNT = 2
  FLAGS = (PADMY,POK,OOK,pPOK)
  IV = 4  (OFFSET)
  PV = 0x98f778 ( "1234" . ) ""\0
  CUR = 0
  LEN = 4