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Occasionally, free can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc.

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PERL_MALLOC_OK
defaults to 0. Should be set to TRUE if the extension can work with the memory allocation routines substituted by the Perl malloc() subsystem. This should be applicable to most extensions with exceptions of those

with bugs in memory allocations which are caught by Perl's malloc();
which interact with the memory allocator in other ways than via malloc(), realloc(), free(), calloc(), sbrk() and brk();
which rely on special alignment which is not provided by Perl's malloc().
NOTE. Negligence to set this flag in any one of loaded extension nullifies many advantages of Perl's malloc(), such as better usage of system resources, error detection, memory usage reporting, catchable failure of memory allocations, etc.