O'Reilly is great if you know the name of the function you want. I find Cross-Platform Perl to be better when you know _what_ you want to do, but can't remember the functions that do it. Titles of chapters are (or were) user-friendly like "Working with Files" or "Launching Applications". O'Reilly has chapters like "References", "Data Structures", "Objects" and my favorite "Functions" (which has 200+ pages of, you guessed it, Perl Functions).
Get this book if you're new to Perl - you won't outgrow it.