my @grid_a; # there are only 9 particles
my @grid_b; # there are lots of particles
for my $x (0..9) {
  for my $y (0..9) {
    for my $particle (0..8) {
      $grid_a[$x][$y][$particle] = 1;
      $grid_b[$x][$y]{$particle} = 1;
    }
    my $pcount = keys %{$grid_b[$x][$y]};
  }
}

use Data::Dumper;
print Dumper(\@grid_a, \@grid_b);