As far as SysV semaphores and shared memory, it's not that they are particularly fast, they're just faster than performing many very small transactions with a database. It may well be that performance is the same as using a small disk file, which should also be pretty fast, as long as the OS is smart about caching and delayed writes. Look at mmap in my updated benchmarks to see fast. :-)
Not sure about portability. I'm using all modules that come with Perl and using only the most basic features, so I would expect it to be at least portable across Unix-like systems. Apart from flock I don't know of another native perl mechanism to do an interprocess mutex.