The reason for this exercise is, of course, to f ind code which uses memory returned by malloc without initializing it and code which uses code after it is freed. valgrind can do this but it's costly to run. The MALLOC_PERTURB_ exchanges the ability to detect problems in 100% of the cases with speed.
The byte value used to initialize values returned by malloc is the byte value of the environment value. The value used to clear memory is the bitwise inverse. Setting MALLOC_PERTURB_ to zero disables the feature.
This technique can find hard to detect bugs. It is therefore suggested to always use this flag (at least temporarily) when testing out code or a new distribution. That's a hint for all the brave souls testing FC6t1.