use warnings;
use strict;

use Data::Dumper;
use JSON;

if (! @ARGV && ($ARGV[0] ne 'first_hash' || $ARGV[0] ne 'second_hash')){
    print "need first_hash or second_hash as arg\n";
    exit;
}

my $want = $ARGV[0];
my $file = 'data.json';

my $json;

{
    local $/;
    open my $fh, '<', $file or die $!;
    $json = <$fh>;
    close $fh;
}

my $data = decode_json $json;

print Dumper $data->{$want};