#!/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 };
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Another possibility is to use traits:

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

{
    package My::Object;
    use Moose;

    has keys => ( is      => 'ro',
                  traits => ['Array'],
                  handles => { add_keys => 'push',
                               get_keys => 'elements' } );

    __PACKAGE__->meta->make_immutable;
}

my $o = 'My::Object'->new;
$o->add_keys('abc', 'def');

say for $o->get_keys;
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($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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