# in the package
sub new {
    my $class = shift;
    # suck in a config, bless and populate $self   
    return $self;
}
sub one_call_only {
    return bless {}, shift;
}
sub attach {
    my $thing  = shift;
    my $self   = ( ref $thing )
        ? $thing
        : $thing->one_call_only();
    # do attach() stuff with $self, and...:
    return $self->{ sub_details };
}