@a_1 = ();
@a_2 = ();
@a_3 = ();   
@a_4 = ();   
@a_a = (\a_4, \a_3);
@a_b = (\@a_2, \a_a);
@a_final = (\@a_1, \@a_b);

#These do work:
       $a_final[0][1][0][0] = "x";
print "$a_final[0][1][0][0]\n";

       $a_final[1][0][0][0] = "x";
print "$a_final[1][0][0][0]\n";

       $a_final[1][2][0][0] = "x";
print "$a_final[1][2][0][0]\n";

# This won't work:
# error message: "Not an ARRAY reference"
       $a_final[1][1][0][0] = "x";
print "$a_final[1][1][0][0]\n";