{
    package FooObj;
    sub new      { bless { _count => 1,
                           _data  => 0 }, $_[0] }
    sub add_data { 
        
        my $self = shift;
        my $data = shift if (@_);

        print "\nCalled: data value is " . ($data || "NONE") . "\n";

        if (defined $data) { die "\nError: data does not look like number\n\n" if ($data !~ /^\d+$/); }

        if ($self->{_count}) {

            $data = 5 if (!defined $data);
            $self->{_data} += $data;
            $self->{_count}--;

            return($self->{_data});

        }
        
    }

}

my $bar = FooObj->new;
while(my $val = $bar->add_data()) {

    # do something here

}