Perl what is the best way to check if an object defined, missing or null within JSON

Question

I have a JSON file below and I want to check 3 states

Within the array "categories" I have another array "children" which is currently null

How can I do to know

1. if children array is null ?
2. if children array is defined and contain at least one data ?
3. if children array is completely missing from the JSON whereas I was expecting to be here

Here below the JSON file

{
  "id": "Store::REZZ",
  "name": "Rezz",
  "categories": [
    {
      "id": "Category::0556",
      "name": "Cinéma",
      "children": []
    },
    {
      "id": "Category::0557",
      "name": "Séries",
      "children": []
    }
  ],
  "images": [
    {
      "format": "logo",
      "url": "https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Google_2015_logo.svg/1920px-Google_2015_logo.svg.png",
      "withTitle": false
    }
  ],
  "type": "PLAY"
}

I tried something but I can manage only the case 1. for others cases I have an "Not a Hash reference" error message

#!/usr/bin/perl
use strict;
use warnings;
use feature 'say';
use JSON qw( decode_json );
use JSON qw( from_json );

# JSON file 
my $json_f = '/home/test';

# JSON text
my $json_text = do {
        open (TOP, "<", $json_f);
        local $/;
        <TOP>
};

my $data = from_json($json_text);

my @tags = @{ $data->{"categories"}{"children"} };
if (@tags) { 
    foreach (@tags) {
        say $_->{"name"};
        say "1. array is ok and contains data";
    }   
} elsif (@tags == 0) {
    say "3. array is empty";
} else {
    say "2. array is missing";
}

__END__

Answer 1

Data::Dumper will let you visualize the perl data structure the JSON is converted to. In your example,

$VAR1 = {
          'images' => [
                        {
                          'withTitle' => bless( do{\(my $o = 0)}, 'JSON::PP::Boolean' ),
                          'url' => 'https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Google_2015_logo.svg/1920px-Google_2015_logo.svg.png',
                          'format' => 'logo'
                        }
                      ],
          'id' => 'Store::REZZ',
          'name' => 'Rezz',
          'categories' => [
                            {
                              'children' => [],
                              'id' => 'Category::0556',
                              'name' => "Cin\x{e9}ma"
                            },
                            {
                              'id' => 'Category::0557',
                              'name' => "S\x{e9}ries",
                              'children' => []
                            }
                          ],
          'type' => 'PLAY'
        };

As you can see from this, $data->{"categories"} is an arrayref of hashrefs, not a hashref itself.

You can iterate over its elements:

foreach my $cat (@{$data->{categories}}) {
    if (!exists $cat->{children}) {
      # No children element
    } elsif (@{$cat->{children}} == 0) {
      # Empty array
    } else {
      # Has at least element in the array
    }
}

Answer 2

1.if children array is null ?

if (!defined($data->{categories})) { ... }

2. if children array is defined and contain at least one data ?

if (defined($data->{categories}) && @{$data->{categories}} ) { ... }

3. if children array is completely missing from the JSON whereas I was expecting to be here

if (!exists $data->{categories}) { ... }