You need to use a sorting algorithm that is "in place".
No. You can simply use external sorting (i.e. you are not restricted to in-place algorithms, especially if you have plenty of free disk space).

Update: On the CPAN, see:

In reply to Re^2: "Out of memory" problem by eyepopslikeamosquito
in thread "Out of memory" problem by abdullah.yildiz

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