has priv_attr => (
	is		=> 'rw',
	isa		=> 'Str',
	reader	=> '_priv_attr',
	writer	=> '_priv_attr'
);

##</code><code>##

sub do_stuff {
	my $self = shift;
	$self->_priv_attr( 1 );
	$self->_priv_attr;
}

##</code><code>##

Attribute (priv_attr) does not pass the type constraint because: Validation failed for 'Str' with value undef at ./test.pl line 22
        Foo::do_stuff('Foo=HASH(0x8d02c18)') called at ./test.pl line 28

##</code><code>##

#!/usr/bin/env perl

use warnings;
use strict;

{
	package Foo;
	use Moose;
	
	has priv_attr => (
		is		=> 'rw',
		isa		=> 'Str',
		reader	=> '_priv_attr',
		writer	=> '_priv_attr'
	);
	
	sub do_stuff {
		my $self = shift;
		$self->_priv_attr( 1 );
		$self->_priv_attr;
	}
	
	1;
}

Foo->new->do_stuff;