{
    my $static_data;  # will persist between calls and IS PRIVATE

    sub do_stuff {

        my ( $arg1, $arg2, $reset ) = @_;

        # first set our static data if we have not been called before
        # if we have been called then $static_data will be defined
        # static_data can not be accessed outside of this block - ie sub

        $static_data = _expensive_or_private_init() if $reset or ! defined $static_data;

        # blah

    }
}