package Lookup;
sub new { # gets some config file };
sub get { warn 'You should implement this method!'; }
sub set { warn 'You should implement this method!'; }

package Lookup::Remote;
use base 'Lookup';
sub get { ... };
sub set { ... };

package main;

use User;
User->init({ source => 'Lookup::Remote', });
my $user = User->new({ id => 1234, });


##</code><code>##

package User;

sub new {
    ...
    $self->{'source'} = $main::USER_LOOKUP_SOURCE;
    ...
}

package main;
use User;

our $USER_LOOKUP_SOURCE = LookupFactory->new('Remote');

my $user = User->new({ id => 1234, });