There are a lot of concepts in any programming language that are not easily searchable. Think operator precedence, how to represent array or hash constants, what happens to the loop variable after the loop.

For all of these concepts I'm always glad I have a perl book beside me. The index at the end of the book and especially the circumstance that all special characters are listed in it, helps tremendously.

I don't see a reason why such an index could not be implemented into the perldoc system, but until someone makes the editorial effort a book index is the fastest way to find your way around a language you haven't been properly introduced to ;-)