I wasn't suggesting that it isn't there. As perrin points out, it's right there in malloc.c at the top of the grep listing I included.

My problem is

1. The almost complete absence of documentation.
2. It's useablilty (or lack thereof).

A little more documentation outlining under what circumstances it can be used--which perl build options are required for it to be enabled etc.--plus some indication of how one might use it would go a long way.

Perhaps because Win32-perl generally translates to AS perl which doesn't appear to be built with the PERL_EMERGENCY_SBRK option enabled, and the facility hasn't been exercised under Win32.

Maybe because sbrk(2) is a *nix system thing and Win32 doesn't have any equivalent?

Try as I might, I can't persuade $^M to do anything, at least not that I can detect.

It is quite difficult to tell as when you have swapping enabled, you can allocate an aweful lot of memory before you run out, but by the time you do, the system has become quite unresponsive, so actually detecting the point of failure is very hard. I tried setting a %SIG{__DIE__} handler that use Carp::croak() to give me a trace back, but I never saw it.