$^M was an Ilya-thing, which he added in during his work on perl's built-in memory allocation system. Nobody else ever really cared about it so far as I remember from p5p, so it suffers some from both neglect and unfinshedness. I remember in the 5.004/5.005 days that it used to work, but I honestly don't remember how, nor whether it ever worked for anyone other than Ilya.