kroach gave the right answer but since it's Perl, partly :P, and since I'd never heard of JSON::Path before this, I want to show the way I would have approached it before learning there was a more native tool (from our own tobyink); with Data::Diver (update to correct oversight: from our own tye) and JSON.

use strictures;
use JSON;
use Data::Diver "Dive";

my $json = <<'';
  { "key" : "value"
   ,"object" : { "elem1": "value1", "elem2": "value2" }
   ,"array" : [ 0, 1 ]
   ,"not_defined" : null }

my $data = decode_json $json;

for my $key (qw/ key object non_exist array not_defined /)
{
    my @exists = Dive( $data, $key );
    printf "%20s -> %s\n",
        $key,
        @exists ? "yeppers" : "nopers";
}

__END__
                 key -> yeppers
              object -> yeppers
           non_exist -> nopers
               array -> yeppers
         not_defined -> yeppers
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