Maybe you want to try the WAIT engine. Trying just 'attachment' in the text query shows Mime-tools at number 5 and Mime::Lite at number 16. A search for 'database' returns the DBI module amongst a host of other database related modules in the first 40 hits. The term 'subject' is so general that I wouldn't expect to get any "meaningful" results from any engine. Both search.cpan.org and kobesearch.cpan.org mention (somewhere on each site) the WAIT engine as well as each other.

As for search.cpan.org, I went and tried 'database' in a documentation seach and DBI came up (along with hundreds of related database modules). It didn't show up in kobesearch.cpan.org (searching on module name or description) presumably because it is below the 100-result limit (alphabetically speaking). Nonetheless, a large number of these do mention the DBI module (right in their NAME-field, shown with the search results), so it is unlikely that even a superficial investigation into 'database' modules wouldn't turn up the DBI module rather quickly.

The short answer is: there are a few cpan search engines out there that do things slightly differently --- it is unlikely that there will be one that satisfies everyone's needs. That's pretty much the long answer too.