use strict;
use warnings;

# A tied hash that dies if you try to insert a key that already exists.
{
	package Tie::Hash::NoOverwrite;
	use Tie::Hash ();
	use Carp;
	use parent -norequire, qw(Tie::StdHash);
	sub STORE {
		my ($hash, $key, $value) = @_;
		# Ignore these packages for the purpose of reporting errors
		local our @CARP_NOT = qw(
			JSON::MultiValueDie
			JSON::Tiny::Subclassable
		);
		croak "Duplicate key: $key" if exists $hash->{$key};
		$hash->{$key} = $value;
	}
}

# A JSON decoder that ties all hashes with Tie::Hash::NoOverwrite.
{
	package JSON::MultiValueDie;
	use parent qw(JSON::Tiny::Subclassable);
	sub _new_hash { tie my %h, 'Tie::Hash::NoOverwrite'; return \%h }
}

my $json = JSON::MultiValueDie->new;

$json->decode(q/  { "a":1, "b":2 }  /);
print $json->error || 'ok', "\n";

$json->decode(q/  { "a":1, "a":2 }  /);
print $json->error || 'ok', "\n";