Also, is there in Windows a way to detect how much heap a perl program has? (I don't think this is a global value.)

Perl will use as much memory as it needs, up to the point the OS is no longer willing to give Perl anymore memory. The usualy cause of the OS no longer willing to give Perl memory is that all other memory has been claimed by other processes. Which is an ever fluctuating value. If a short-lived process uses all memory of the computer, even for a brief moment, and Perl asks for a small slice of memory just at the same time, Perl will die with an out of memory error. Because it couldn't get the memory from the OS at the moment it wanted.

So, there's no way to find out how much memory Perl can use. Because it's a value that can change on every clock cycle.