use Devel::Peek;
$Devel::Peek::pv_limit = 1;

for ( 0 .. 2 ) {
    my $s = 'a' x 1e6;
    substr $s, 499_998 + $_, 2, 'b';
    Dump $s;
}

__END__

SV = PV(0xdbb198) at 0x26f8ae0
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  OFFSET = 1
  PV = 0x2954309 ( ""... . ) "a"...\0
  CUR = 999999
  LEN = 1000001
SV = PV(0xdbb198) at 0x26f8ae0
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  OFFSET = 1
  PV = 0x2954309 ( ""... . ) "a"...\0
  CUR = 999999
  LEN = 1000001
SV = PV(0xdbb198) at 0x26f8ae0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x2954308 "a"...\0
  CUR = 999999
  LEN = 1000002