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Asked: 2026-05-23T23:00:37+00:00

I am trying to trigger the hash randomization feature in Perl 5.8.2 and later.

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I am trying to trigger the hash randomization feature in Perl 5.8.2 and later. To do this, I need a set of keys that would be pathological but for the randomization feature. I tried using MJD's code, but that didn’t work. And, now that I look at it, I shouldn’t be surprised. It comes from 1997, and we have switched hashing functions since then.

Does anyone know of a set of pathological keys, or how to generate them? I guess I should just go look at the hash function and reverse engineer a solution, but I am lazy.

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  1. Editorial Team

    Note: this information is valid for perl between 5.8.2 and 5.17.x. Beginning with perl 5.18, perl uses a new hash algorithm, hash randomization occurs on every startup (instead of only when a pathological hash is detected), and PERL_HASH_SEED is interpreted differently. As perl now uses a 128-bit hash seed, the “pathological hash” condition should be almost impossible to trigger intentionally.

    Original answer: If you force the hash seed to 0 by putting PERL_HASH_SEED=0 in the environment before perl starts, the collection of keys "\0", "\0\0", "\0\0\0", etc. will cause every key to land in hash bucket 0 with current perls.

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