This compiletime flag affects the way perl interacts with the putenv() libc call.

Without it, subtle memory corruption happens if both perl and some other C library attempt to putenv() within the same program. This corruption is only detected at destruction time, and only on thready+debugging builds.

An example of this can be seen in https://rt.cpan.org/Ticket/Display.html?id=140138

I believe the solution is simply that Perl should build using this flag all the time.