Moose/mouse r/o variables

I have a small class, where I would like the variables to be read only
by external calls, but rw for internal access.

Why? 
If there is a variable defined as lazy and ro, writing the value fails.
So it needs to be rw. Is there a recommended way of dealing with this
problem?

The other reason I have is that the class in question is instantiated
with a filename, and the filename must exist. If the file gets deleted
before the default for value is performed, I would like to get an error.
The easiest way that I found for that was to read and write the filename
value, forcing the isa FileExists. Is there a better way of doing this? 


subtype 'FileExists'
    => as 'Str'
    => where { -e $_ };

has 'fileName' => (is => 'rw',
		   required => 1,
                   isa => 'FileExists',
		  ) ;

has 'value'   => (is => 'rw',
	       lazy => 1,
	       builder => 'value_builder',
	      ) ;

sub value_builder { 
    my $my = shift ;

    $my->fileName($my->fileName) ; # This forces the isa requirement
    $my->value('123') ;	
} 

Peter