Got it. Simple (and obvious!). For any future readers, this is how the example code would be modified:

#!/usr/bin/perl

use XML::RSS;

my $description = "<b>How do I get this to be CDATA?</b>";

my $rss = new XML::RSS (output=>'2.0');

$rss->channel(
        title           => "Sample RSS Feed",
        'link'          => "http://www.somewhere.com",
        description     => "Example Feed",
);

$rss->add_item(
    title   => "Test",
    'link'  => "http://www.testitem.com",
    description => "<![CDATA[$description]]>",
    
);

print $rss->as_string();
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