Use a tie to your array or hash, so it lives in a file (database?) and swaps in transparantly.

There are already database/hash tie modules, but it might not cache the way you want.

sub STORE {
 my ($self, $index, $value)= @_;
 ... if index is in range, store item in my array
 ... else store it to the file.
 ... if I appended more elements and hit my high-water
 .... mark, swap out lower items and adjust my range.
 }
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