package Foo {
    use Moo;

    has qux => (is => 'ro');

    around BUILDARGS => sub {
        my ($orig, $class, $args) = @_;
        return {
            qux => $args->{bar} * $args->{baz},
        };
    };
};

use strict;
use warnings;
use feature 'say';

my $o = Foo->new({
    bar => 6,
    baz => 7,
});

say $o->qux;
say $o->bar;

##</code><code>##

$ perl 11107488.pl

42
Can't locate object method "bar" via package "Foo" at 11107488.pl line 24.