Have you printed $mech->content? I assume that Firefox only supports CSS selectors for HTML. The page you gave is RSS (or ATOM or whatever other feed format). I assume that Firefox does not want to run CSS queries against RSS. Replacing your call to ->click with a simple call that returns the elements shows that no elements get found for either ->selector or ->xpath.

use WWW::Mechanize::Firefox;

my $firefox = WWW::Mechanize::Firefox -> new (
);

$firefox -> get("http://feeds.cbsnews.com/podcast_eveningnews_video_1?
+tag=contentMain%3bcontentBody");
print $firefox->ct;

print $_->{innerHTML} for $firefox->selector('a');

print $_->{innerHTML} for $firefox->xpath('//a');
[download]

I'm not sure what goes wrong here, but I assume that Firefox does not really support CSS or XPath queries for documents other than HTML documents. Even if it did, I highly doubt that the surrounding event model would support the ->click event as HTML pages do.

You can consider looking at Mojolicious, Web::Magic, Web::Scraper or App::scrape, all of whom do not munge the RSS through Firefox and all of whom support CSS and XPath queries - maybe you have more luck extracting the URLs with those.