That's not a sparse array. It has 10,000,001 allocated slots, all of which are in use.

You make "keeping track of valid values" sound very complicated but it's just 3 numbers.

>perl -MDevel::Peek -e"$a[6]=1; $a[7]=1; shift(@a); Dump(\@a,1)"
SV = IV(0x1acf60) at 0x1acf68
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x336af0
  SV = PVAV(0x1adc30) at 0x336af0
    REFCNT = 2
    FLAGS = ()
    ARRAY = 0x330590 (offset=1)    -- Address of first element
    ALLOC = 0x330588
    FILL = 6                       -- Index of last element
    MAX = 12                       -- Index of last allocated element
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

The array has 7 elements and 14 allocated elements (-1..12 relative to ARRAY).

Update: Replaced description with an example.