To make full (and "correct") use of OO, the properties need to be encapsulated.

use warnings;
use strict;

package CfgBase {
    use Moo;
    use namespace::clean;
    has on_error    => (is => 'ro', default => "exit");
    has output_type => (is => 'ro', default => "text");
    sub dump {
        my $self = shift;
        print ref($self),":\n";
        print "  $_: ",$self->$_,"\n" for qw/ on_error output_type /;
    }
}

package CfgChild {
    use Moo;
    use namespace::clean;
    extends 'CfgBase';
    has '+on_error' => (default => "warn");
}

CfgBase->new->dump;
CfgChild->new->dump;
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