You may have suggested this just because you were thinking of mushrooms (mushrooms!), but when I was trying to come up with what animal fit Perl, badger worked. It isn't known for how fast it can run, yet it can rip through problems and doesn't follow mainstream language design fads. *shrug*
Of course, O'Reilly already has a "badger book".
Which brings me back to the black pearl in an oyster.
Yay, memes! But now that I think of it, I kinda like the idea of a badger, (or maybe a wolverine, similar concept) and, like, they're kinda cute and fun, as long as they don't rip your face off in the process <g>.
And while I can certainly understand there being issues regarding the usage of the Camel as Perl's mascot, I wouldn't think there would be issues with using some other animal (which O'Reilly may already have on a cover), because, as I understand it, the issue with the camel, is using it in connection with Perl specifically, I don't imagine they could prevent the usage of all the animals they have on their covers (except in connection with the books topics) else there would be copyright issues when anybody uses just about any animal as a mascot... And anyways, if we go by that, what'll we be left with, maybe a Mule and a Jackal?