It looks like it is from "Beginning Perl" by Curtis Poe. Page 300 has the whole program, page 301 discusses is_prime() which is non-optimized trial division (checks divisibility by all integers up to sqrt(n)). Page 302 discusses the code in question which is introducing using a hash to cache the result.

In particular, the test suite includes duplicates in its list. That's why the hash is used.

In reply to Re^3: Unable to Understand grep and hash use for prime no. by danaj
in thread Unable to Understand grep and hash use for prime no. by shankonit

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