use Tie::Array::Packed;

tie my @n, 'Tie::Array::Packed::Number';
push @n, rand for 1..30_000_000;
tied(@n)->sort;
for (0, 25, 50, 75, 100) {
  printf "%03d%%: %6.4f\n", $_, $n[$#n*$_/100];
}
[download]

That runs in one minute in my computer