If I look at my Konqueror bookmarks.xml file, I see:

<!DOCTYPE xbel>
[download]

One quick Google later and I've found documentation for the Xbel bookmark format. A scan of CPAN and I've found XML::XBEL.

On the other hand, if I poke at my Firefox bookmarks.html file, I see:

<!DOCTYPE NETSCAPE-Bookmark-file-1>
[download]

So Netscape::Bookmarks is probably better for reading and writing in this format.