indispensable... even more necessary than the camel imho...
i couldn't fathom how much time and page-flicking i've saved since owning it - the 'recipe' format is remarkably intuitive and oh-so appropriate for a reference book. An excellent book to learn perl with too, if you already know how to program.
yet another o'reilly must-buy (does o'reilly publish any books which aren't good or excellent?)

matt harrison,
Biochem PhD student, and late-night nouveau-hacker