There are two macros which serve as bulk disablers of advanced
  features of this malloc: NO_FANCY_MALLOC, PLAIN_MALLOC (undef by
  default).  Look in the list of default values below to understand
  their exact effect.  Defining NO_FANCY_MALLOC returns malloc.c to the
  state of the malloc in Perl 5.004.  Additionally defining PLAIN_MALLOC
  returns it to the state as of Perl 5.000.