I have an experience that perl's own memory allocator is pretty good. I am curious why it is not *on* by default.

Apart from thread safety, there is the issue of "bug compatibility" with the system malloc when using buggy third party libraries. I've seen cases where third party libraries erroneously call free on a pointer that was not got from malloc, yet the system malloc is forgiving and does not crash. Now if you replace the system malloc with a pickier one that does crash ... In one way, that is good because you've just flushed out a bug in the third party library. OTOH, you may not have the source code for the third party library to fix it and the vendor may be unresponsive. Most third party libraries are tested primarily with the system malloc and work more reliably with it.

See also Using Perl's Malloc.