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Probably for the same reason that it takes a little more memory to create an identical array the second time than it did the first:
When building an array piecemeal this way, the base AV starts with 8 elements, therefore requires 64 bytes contiguous memory be allocated. When you add the 9th element to it, the AV has to be reallocate to accommodate that new element and so a new chunk of memory double the size (128 bytes) is allocated; the existing 8 elements are copied across and the new 9th element is added (leaving space for 7 more new elements). Then and the first AV is freed back to the memory pool.
Then when you go to add the 17th element; the size is doubled again (256 byte AV is allocated) and the 128 are freed back to the pool. This process of doubling continues, until the size of the size of the AV required gets bigger than the pool limit, and then rather than allocating the newly doubled AV from the pool; it is allocated from the OS.
During the process of creating the array the first time, the pool is expanded -- by requesting more memory from the OS -- to accommodate the AVs when they are less than the size where they get allocated directly from the OS. And when they are done with, those allocations are returned to the pool and will be reused for other allocations including the many 24-bytes SVs. by the time the first array is freed, the once contiguous chunks have been re-allocated to smaller subdivisions.
Hence, when the array is built the second time, the continuous chunks originally allocated from the OS to accommodate the intermediate AVs are no longer available as contiguous chunks, so as the array is built the second time, it is necessary to go back to the OS for new contiguous allocations, despite that the final array will be the same size as the first.
Hashes go through similar processes as they expand.
As XML::Simple builds nested structures of hashes and arrays, it has similar requirements for contiguous chunks of memory, that may need to be reallocated when re-building the same data-structure a second time.
(Note for the pedants: The above may not be a totally accurate description of the process; but it is sufficient to explain the point I am trying to convey.)
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In reply to Re^3: How can I free the memory of a XML::Simple-Object