use v5.14;

package Part {
	use Moose;
	has foo  => (is => 'ro');
	has bar  => (is => 'ro');
}

package Whole {
	use Moose;
	has baz  => (is => 'ro');
	has part => (
		is       => 'ro',
		writer   => '_set_part',
		init_arg => 0,
		isa      => 'Part',
		handles  => qr/.*/,
	);
	
	sub BUILD {
		my $self = shift;
		$self->_set_part(Part->new(@_));
	}
}

my $obj = Whole->new(foo => 1, bar => 2, baz => 3);

print $obj->dump;

say for $obj->foo, $obj->bar, $obj->baz;