In my simple tests, the SV is being reused, but that isn't the space I am concerned about being reused. The hope ws that the PVAV used by the array and the PV used by the bitvector would reuse the same space, but that isn't the case here.

use Devel::Peek;;
my @a = 1 .. 87382; Dump \@a; <STDIN>; 
undef @a; <STDIN>; 
my $b = 'x'x2**20; Dump \$b; <STDIN>;

SV = RV(0x183573c) at 0x1cd99fc
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x195d6d0
  SV = PVAV(0x195cb7c) at 0x195d6d0
    REFCNT = 2
    FLAGS = (PADBUSY,PADMY)
    IV = 0
    NV = 0
    ARRAY = 0x1d2fbbc
    FILL = 87381
    MAX = 87381
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x194246c) at 0x195d82c
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 1
    Elt No. 1
    SV = IV(0x1942468) at 0x195d814
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 2
    Elt No. 2
    SV = IV(0x1942470) at 0x1824704
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 3
    Elt No. 3
    SV = IV(0x1942474) at 0x195d7fc
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 4


SV = RV(0x183573c) at 0x195d82c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x1cd999c
  SV = PV(0x225584) at 0x1cd999c
    REFCNT = 2
    FLAGS = (PADBUSY,PADMY,POK,pPOK)
    PV = 0x203002c "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx...
    CUR = 1048576
    LEN = 1048577
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There is also the problem that when the function is called, there would be no way to know what amount of reusable memory is alredy available in the system.

On win32 I can query the VM size directly from the OS and form my upper bound from that, but I have no idea how to make that cross system compatible. Perl almost certainly has a good idea of where the end of it's memory is. I just don't have any clues how I could tap into that information?