Please try to reduce your code down to the minimum needed to reproduce the issue - see SSCCE. Like this:

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 {}
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The problem is that $foo->Fields is a method call, to which you cannot assign anything ($foo->Fields = ...;) by default; it's as if you had written sin($pi) = 123;. The normal way to set an object property is to pass its new value as an argument to the method, as I showed above: $foo->Fields({});

There is a way to make methods that you can assign to in Perl, Lvalue subroutines, but at least in my experience they seem to be pretty uncommon. I suggest you stick to normal setter calls.

Also, let me repeat Mr. Muskrat's suggestion to look into DBIx::Class instead of reinventing that wheel.