perlop notes some differences b/w autoinc and autodec
I'm not sure if it has any bearing on this...

The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the vari- able has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern "/^a-zA-Z*0-9*\z/", the increment is done as a string, preserving each character within its range, with carry:

The auto-decrement operator is not magical.