You can make your application reads the configuration and store it in some place your User class knows.

I wrote an application that has a %config all my modules can access (using Config::IniFile). I created sections for all modules and grab configuration from there.

Config example:

[User]
lookup_uri = something
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Then your code could be like this:

package App;
our %config = ();
&init;
sub init {
    # loads your configuration file/parameters here and
    # assigns to %App::config
}

package User;

sub new {
    my $self = {};
    bless $self;
    $self->{lookup_uri} = $App::config{lookup_uri};
}
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You can also specify which class you want to use for Lookup in config file, and instantiate in User::new.

Still hope it helps :)