MetaCPAN uses the PAUSE generated 02packages.details.txt file. If it's not in there, then the module author will need to fix this. (Authors are usually emailed about this when they upload a distribution).
A distribution must contain a package, even if it is 100% POD, a good example of this done correctly is Catalyst::Manual. Check the package declaration in the source of this file. That's the key to getting your pure POD distribution indexed by PAUSE.
search.cpan.org has evolved to support edge cases or have exceptions but as the code is not public we are unable to tell you what they are doing.
We do have bugs, so this is possible, please help by...