You have to specify where the User module will load its data. You can also use something like this: 
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, });

[download]
Or you can even tell User constructor to check some value in main scope: 
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, });

[download]
You still have to inform the User class what class it will use to get data. I can be wrong, that's the reason I come to perlmonks :)

Update: I think it is like DBI specs. I don't know what is going under the DBD module, but I have to inform DBI where the data is.

In reply to Re^3: OO, Library Configuration and Class Variables by izut
in thread OO, Library Configuration and Class Variables by moot

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