As far as other languages go: C, bash, SQL, Makefile, and autoconf/automake. That isn't as much as it sounds like -- learning bash will come pretty naturaly. Makefile/autoconf/automake you only really need to know enough of to debug problems, and hopefuly fix them. (BTW, a hint: reading the files generated by auto(conf|make) is diffucut, and normaly not useful.) C you should be able to at least read with fair fluency. Personaly, I learned most of my C from reading, and occasionaly hacking on, the Linux kernel.

Oh, and it really is always possible to do everything in perl. I've even kind of written device drivers in it, with some Inline::C, and without integrating with the kernel. (IE, not device-drivers-proper, but debugging register dumpers and such.)