No can do. I don't know why.

Update: Actually, it's quite basic. The assignment of do's return value causes it to be copied. You'll notice the same effect from subroutines.

use Devel::Peek qw( Dump );

{
   my $x = do {
      my $y = 'abcdef';
      Dump($y);
      $y
   };
   Dump($x);
}

{
   my $x = sub {
      my $y = 'abcdef';
      Dump($y);
      $y
   }->();
   Dump($x);
}

__END__
SV = PV(0x226df4) at 0x226ce0
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x1822b04 "abcdef"\0
  CUR = 6
  LEN = 8
SV = PV(0x226e24) at 0x226d34
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x187a93c "abcdef"\0           <-- new buffer
  CUR = 6
  LEN = 8
SV = PV(0x226e0c) at 0x1830320
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x182c84c "abcdef"\0
  CUR = 6
  LEN = 8
SV = PV(0x226e60) at 0x226d70
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x1855fac "abcdef"\0           <-- new buffer
  CUR = 6
  LEN = 8
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