use warnings;
use strict;
use Data::Dump;
{
	package Foo;
	use Moose;
	has 'Fields' => (is => 'rw', isa => 'HashRef');
}
my $foo = Foo->new(Fields=>{});
$foo->Fields->{x}++;
dd $foo->Fields;  # prints { x => 1 }

#$foo->Fields = 0;  # wrong, fails
$foo->Fields({});
dd $foo->Fields;  # prints {}