It's a version of 
@primes = grep { is_prime($_) } @numbers;

[download]
that caches the results of calls to is_prime to avoid calling it more than once with the same input. It trades off memory in an attempt to speed up the process.

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

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