package Number;
use overload
   [snipped]
    '""' => sub { ...; };
##</code><code>##

### example package

package Number;

my $self = {};

sub new {
my $class = shift();
$self->{decimal} = shift();
return bless( $self, $class );
}

my @roman_numbers = qw[
  undef I II III IV V VI VII VIII IX X
];

use overload
  '""' => sub { return $roman_numbers[ $self->{decimal} ] };
1;

### main

package main;
my $instance = new Number(3);
print $instance;

### it will print 'III'