Thanks for clarification. I.e. we better be careful with 4-args (or LHS) variant of substr?

It's funny, Perl rightfully decides (case "0") it's cheaper to move 499_998 initial bytes _up_ than moving the trailing 500_000 bytes _down_ (and so scalar gets OOK-"contaminated"); and it's the opposite (rightfully) for "2" -- scalar stays OOK-free & usable OK with regex engine. When ("1") they are both 499_999, then... -- ??? move the 1st half UP and actually _complicate_ things! (do additional work of setting the OFFSET?)

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
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