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

{
    package MyStruct;
    use Moose;

    has fields => ( is => 'rw' );

    __PACKAGE__->meta->make_immutable;
}

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