MacPerl is for Mac OS. Not for Mac OS X. Really, the only reason you'd want to use it for Mac OS X is if you want to use the Mac:: modules that are (currently!) specific to MacPerl/Mac OS, that don't yet work on Mac OS X.
Once those modules largely work on Mac OS X, there will be no reason at all to use MacPerl on Mac OS X, I'd wager.
However, many people still use Mac OS, and haven't yet or never will switch to Mac OS X. MacPerl is primarily for them. :-)