use JSON;
use Data::Dumper;
use strict;
use warnings;

my $json = JSON->new->allow_nonref;


my $raw = do { undef $/; <DATA> };
my $ds = $json->decode( $raw );

if(0)
{
# this replaces the value of 'genres' with a hash more like what you w
+ant:
for my $key ( keys %$ds ) # e.g. 'genres'
{
    my %h;
    for my $r ( @{ $ds->{$key} } )
    {
        $h{$r->{'id'}} = $r->{'name'};
    }
    $ds->{$key} = \%h; # replace it!
}
print Dumper $ds;
}

# or if you really want to 'invert' the hash, so that 'id's are at the
+ top:
my %data;
for my $key ( keys %$ds ) # e.g. 'genres'
{
    my %h;
    for my $r ( @{ $ds->{$key} } )
    {
        $data{$r->{'id'}}{$key} = $r->{'name'};
    }
}
print Dumper \%data;


__DATA__
{
  "genres": [
    {
      "id": 28,
      "name": "Action"
    },
    {
      "id": 12,
      "name": "Adventure"
    },
    {
      "id": 16,
      "name": "Animation"
    },
    {
      "id": 35,
      "name": "Comedy"
    },
    {
      "id": 80,
      "name": "Crime"
    },
    {
      "id": 99,
      "name": "Documentary"
    },
    {
      "id": 18,
      "name": "Drama"
    },
    {
      "id": 10751,
      "name": "Family"
    },
    {
      "id": 14,
      "name": "Fantasy"
    },
    {
      "id": 36,
      "name": "History"
    },
    {
      "id": 27,
      "name": "Horror"
    },
    {
      "id": 10402,
      "name": "Music"
    },
    {
      "id": 9648,
      "name": "Mystery"
    },
    {
      "id": 10749,
      "name": "Romance"
    },
    {
      "id": 878,
      "name": "Science Fiction"
    },
    {
      "id": 10770,
      "name": "TV Movie"
    },
    {
      "id": 53,
      "name": "Thriller"
    },
    {
      "id": 10752,
      "name": "War"
    },
    {
      "id": 37,
      "name": "Western"
    }
  ]
}
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