sub server {
    my $self = shift;
    ...
}

##</code><code>##

my $payer = Business::OnlinePayment->new();
set_defaults($payer);

##</code><code>##

sub Business::OnlinePayment::set_defaults {
  ... your code here ...
}

##</code><code>##

my $payer = Business::OnlinePayment->new();
$payer->set_defaults();