sub do_stuff
{
    my $this = shift;
      my $session = shift 
           || die "Missing req'd Session argument";
      my $cgi  = shift || new CGI ();
       ...
}

##</code><code>##

sub do_stuff {
    my ($self, $session, $cgi) = @_;
    croak 'Session not optional' unless $session;
    $cgi ||= CGI->new();

    ...
}