I've never had much success with searching CPAN by functionality - often I want something specific which may not appear in the module name or description (I don't know if search.cpan.org even looks at descriptions).

A better approach, I've found, is to look for previously asked questions by searching for likely phrases on various sites (Perlmonks being foremost). Example: I want a module that does X, search for "how do I do X", "problem doing X", "doing X doesn't work" (more or less, allowing for proper grammar etc. depending on the value of X).

This usually shows up code with use statements or module recommendations or (best case) admonitions to "just use Some::Mod, you'll want to eventually anyway" (WWW::Mechanize for most web client stuff would be an example of the latter).