I looked into it for a sec., I already had problems with XML::Atom too. At least the error goes away if you pass an URI object into the constructor of XML::Atom::Feed:

#!/usr/bin/perl -w

use strict;
use URI;
use XML::Atom::Feed;

my $feed = XML::Atom::Feed->new(URI->new ('http://mildlydiverting.blog
+spot.com/atom.xml'));

for my $entry ($feed->entries()) {
       print $entry->title()."\n";
}
[download]

UPDATE: I spent some more time with it, but I couldn't find out more in the limited time. I suggest you either install LibXML or try to report the bug via rt.cpan.org at XML-Atom on CPAN, or mail it directly to the author (which maybe has a higher chance of success :)