my_cache.pl

sub get_data
{   
    my ($self, $key) = @_;
    $data = $self->get($key);
    $flag = $self->get("$key\::flag");
    unless ($flag && $data) {
        $self->set("$key\::flag", 1, $self->expire - $self->delta);

        ...; # do task

        $self->set($key, $data, $self->expire);
    }
    return $data;
}