The key concept that salva exploits in his excellent example is the "offset OK hack". When you call substr in Perl and lop a few characters off the front of a scalar... instead of reallocating and copying, Perl performs the following steps:

1. Store the number of bytes to get lopped in the scalar's IV.
2. Move the SvPVX pointer forwards.
3. Set the scalar's OOK flag.
4. Turn off the scalar's IOK flag.
5. Adjust the scalar's CUR and LEN to reflect the change.

$ perl -MDevel::Peek -e \
'my $toes = "potatoes"; substr($toes, 0, 4, ""); Dump($toes);'
SV = PVIV(0x1801a20) at 0x1801434
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,OOK,pPOK)
  IV = 4  (OFFSET)
  PV = 0x300b64 ( "pota" . ) "toes"\0
  CUR = 4
  LEN = 5
[download]

See the section "Offsets" in perlguts for a thorough explanation, as well as the sv_chop function in perlapi. Perl_sv_chop in the sv.c source code is only a few lines long, so you might also want to snoop that.