#!/usr/bin/perl
use Modern::Perl;

{
    package Foo;
    use Moose;

    has bar => (
        is     => 'ro',
        writer => '_set_bar',
    );

}

my $foo = Foo->new;

$foo->bar('something');

# outputs:
# Cannot assign a value to a read-only accessor at
# /home/brunov/tmp/pm.pl line 16

$foo->_set_bar('something');
say $foo->bar;

# outputs:
# something