#$hash{$array1[$_]}{$array2[$_]} = $array3[$_] for (0..$#array1);
push @{$hash{$array1[$_]}{$array2[$_]}},  $array3[$_] for (0..$#array1);

##</code><code>##

OUTPUT:
$VAR1 = {
          '400000008' => {
                           'Network1' => [
                                           '2021-01-25T17:19:36.000Z'
                                         ]
                         },
          '500000008' => {
                           'Network2' => [
                                           'Not Used'
                                         ],
                           'Network1' => [
                                           '2021-03-10T17:11:24.000Z',
                                           '2021-03-11T17:12:42.000Z'
                                         ],
                           'Network3' => [
                                           'Not Used'
                                         ]
                         },
          '900000001' => {
                           'Network1' => [
                                           'Login'
                                         ]
                         },
          '900000003' => {
                           'No Network1' => [
                                              '   NOT Registered'
                                            ]
                         },
          '900000005' => {
                           'Network1' => [
                                           'Login'
                                         ]
                         },
          '900000007' => {
                           'Network1' => [
                                           '2021-03-10T17:11:24.000Z',
                                           '2021-02-18T08:34:58.000Z',
                                           '2021-01-25T17:19:44.000Z'
                                         ]
                         },
          '900000002' => {
                           'Network1' => [
                                           '2019-11-07T20:31:10.000Z',
                                           '2021-02-19T13:47:25.000Z',
                                           '2021-03-10T13:58:50.000Z'
                                         ]
                         },
          '900000004' => {
                           'Network1' => [
                                           'Login'
                                         ]
                         },
          '900000006' => {
                           'Network1' => [
                                           'Login2'
                                         ]
                         }
        };