I guess File::Spec's API is sane enough, but I suspect not a lot of people are using it because there's not enough incentive for it. When 99% population of the world use Unix/Linux/Windows (even Macs been technically Unix for a number of years), "/" works everywhere and using File::Spec does not gain you anything except lots of typing exercise.
That's why I think Path::Class might have a better chance of succeeding. It gives niceties like a few more convenience methods, a shortcut of getting dir & file object from each other, etc. It gives users more incentive of using a proper path manipulation library because it gives extra stuff along with that. It should also be in core to accompany File::Spec.
One of the standard libraries in the perl tool belt, File::Spec is what you will want to use to chop up and concatenate paths portably. Don't assume forward slashes will just work everywhere and paste stuff together by hand, use File::Spec. Very useful module.