perl -MData::Dumper -e "$hash{lvl1}[3]{lvl3}[0] = 42; print Dumper \%hash"

$VAR1 = {
          'lvl1' => [
                      undef,
                      undef,
                      undef,
                      {
                        'lvl3' => [
                                    42
                                  ]
                      }
                    ]
        };