use Devel::Peek qw( Dump );
my $s = "abcdef";
$s .= "g";  # Trigger COW
Dump( $s );
substr( $s, 0, 1, "" );
Dump( $s );

##</code><code>##

SV = PV(0x59d752a54ee0) at 0x59d752a830d0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x59d752a6b2e0 "abcdefg"\0
  CUR = 7
  LEN = 16
SV = PV(0x59d752a54ee0) at 0x59d752a830d0
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  OFFSET = 1
  PV = 0x59d752a6b2e1 ( "\x01" . ) "bcdefg"\0
  CUR = 6
  LEN = 15