In your example, you don't want the <rss> tag to be in the itunes: namespace, so you shouldn't be passing in a namespace to the corresponding XML::Generator call. All you need to do to get the xmlns:itunes attribute into the <rss> tag is to use that namespace in some embedded content. For example:
$itunes = XML::Generator->new(namespace=>[itunes=>'http://...']);
$gen = XML::Generator->new();
print $gen->rss($itunes->author("Some Guy"))
This generates:
<rss xmlns:itunes="http://..."><itunes:author>Some Guy</itunes></rss>
