package MyObj;

use Moose;
use namespace::autoclean;

# some irrelevant attributes and methods...

has 'custom_fields' => (
	traits  => [qw( Hash )],
	isa     => 'HashRef',
	builder => '_build_custom_fields',
	handles => {
		custom_field        => 'accessor',
		has_custom_field    => 'exists',
		custom_fields       => 'keys',
		has_custom_fields   => 'count',
		delete_custom_field => 'delete',
	},
);

sub _build_custom_fields {
	return {};
}
__PACKAGE__->meta->make_immutable;

1;

##</code><code>##

my $obj =  MyObj->new();
$biorange->custom_field('foo', 23); # OK
my $foo = $biorange->custom_field('foo'); # OK
my $foo = $biorange->custom_field('bar'); # should croak!