package Krautcat;
use strict; use warnings;
use Moo;
use MooX::TypeTiny;:
use Types::Standard qw/ Str is_Str /;

has foo => ( is => 'ro', isa => Str, required => 1 );
has baz => ( is => 'rw', );

sub qux {
    my $self = shift;
    my $val = shift;

    die 'not a string' if not is_Str $val;
    $self->baz( $val );

    return { baz => $self->baz };
}

1;