perl -MData::Dumper -E '$A[3][5][0]="foo"; die Dumper \@A'

output:

$VAR1 = [
          undef,
          undef,
          undef,
          [
            undef,
            undef,
            undef,
            undef,
            undef,
            [
              'foo'
            ]
          ]
        ];
[download]

Long? Just try 300 by 500!

perl -MData::Dumper -E '$A{3}{5}{0}="foo"; die Dumper \%A'

output:

$VAR1 = {
          '3' => {
                   '5' => {
                            '0' => 'foo'
                          }
                 }
        };
[download]

smaller, but slower to iterate through if you have lots of entries. Still you can use an intermediate variable that acts like a pointer:

perl -MData::Dumper -E '$A{3}{5}{0}="foo"; $v = $A{3}{5}; $v->{1}="bar
+"; die Dumper \%A'
[download]

if you have fixed dimensions... you can also try: $NUM = $x + $y*$WIDTH so if you have 300 pixels wide, (x,y)=(3,5) becomes 3+5*300 = 1503

but check if you will not run above your maxint: 718414 with that method