Without intending to deprecate your question, were I allowed to own only one Perl book ...

I'd be in deep trouble!

clinton's point captures much of my reasoning, but FWIW, it took a book not mentioned, SAMS' "Perl Developer's Directory" to offer an explanation I could comprehend/grok/absorb of a point covered all too briefly (for me) in the docs and the (generally excellent) O'Reilly series.

We tend to think of TIMTOWTDI as applied to coding, but it also applies to learning.