Not sure why they decided not to implement -- on chars
Not every computational process is reversable, nor should it be. "clintp"-- probably would go to "clinto" you'd think.
"aa"-- might even logically go to "z", but then where should "a"-- go? "-a"? undef? ""? "`"? (how about on ebcdic systems?) Throw a warning (eeek! no warnings 'autodecrementascii'), throw an error (not remotely perlish!)?
Sometimes language features are nice, you acknowledge them, maybe use them, move on, and not think too hard about making them fit into every conceivable programmatic niche you can think of. Perl's gotten in trouble for this kind of nonsense before (pseudohashes, v-strings). Enjoy magical autoincrement. And remember putting magic under a microscope takes all of the joy out of it.