#!/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};
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Update: Another option is to use traits:

#!/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 $s->get_fields('b');
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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