System V IPC
Everyone hates System V IPC. It's slower than paper tape, carves out insidious little namespaces completely unrelated to the filesystem, uses human-hostile numbers to name its objects, and is constantly losing track of its own mind. Every so often, your sysadmin has to go on a search-and-destroy mission to hunt down these lost SysV IPC objects with ipcs (1) and kill them with ipcrm (1), hopefully before the system runs out of memory.
Despite all this pain, ancient SysV IPC still has a few valid uses. The three kinds of IPC objects are shared memory, semaphores, and messages. For message passing, sockets are the preferred mechanisms these days, and they're a lot more portable, too. For simple uses of semaphores, the filesystem tends to get used. As for shared memory--well, now there's a problem for you. If you have it, the more modern mmap (2) syscall fits the bill, but the quality of the implementation varies from system to system. It also requires a bit of care to avoid letting Perl reallocate your strings from where mmap (2) put them. But when programmers look into using mmap (2), they hear these incoherent mumbles from the resident wizards about how it suffers from dodgy cache coherency issues on systems without something called a "unified buffer cache"--or maybe it was a "flycatcher unibus"--and, figuring the devil they know is better than the one they don't, run quickly back to the SysV IPC they know and hate for all their shared memory needs.