Well, that quite depends on the setup. Many Linux distros use one big partition by default anyway (or at least have /usr on the root mount), so once you have anything, you have perl (assuming it's in /usr/bin/perl). And traditionally, the root mount contained /bin with just a small set of programs, just enough to get to the stage to mount other file systems. Anything interesting would be in /usr/bin anyway. Furthermore, mounting other file systems happens pretty early in the boot process anyway - so most rc scripts will have perl available, even if perl is not on the root mount.
But I've also worked for a company where we used Linux boxes that only had 2 Mb of memory, 25 Mb disks, and for which the OS had to be installable from a single floppy disk. Needless to say, said boxes didn't have Perl, although I used Perl a lot to create the distros.