#!/usr/bin/perl
use warnings;
use strict;

{
    package MyStruct;
    use Moose;

    has fields => ( is => 'rw',
                    isa => 'HashRef',
                    traits => ['Hash'],
                    handles => {
                        set_fields => 'set',
                        get_fields => 'get',
                    },
                  );

    __PACKAGE__->meta->make_immutable;
}

my $s = 'MyStruct'->new(fields => { a => 0, b => 1 });
$s->set_fields(b => $s->get_fields('b') + 1);
print $s->get_fields('b');