#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

{
    package My::Object;
    use Moose;

    has keys => ( is      => 'ro',
                  isa     => 'ArrayRef',
                  default => sub { [] } );

    __PACKAGE__->meta->make_immutable;
}

my $o = 'My::Object'->new;
$o->keys->[0] = 'def';
push @{ $o->keys }, 'abc';

say for @{ $o->keys };