The node Truth and Falsehood also briefly discusses this. In short, a hash in boolean context returns a false value when it is empty, so if (!%hash) will portably detect an empty hash. To portably get the number of keys in a hash, use keys in scalar context; scalar(%hash) is not backwards compatible for counting keys (as the documentation quoted by LanX explains).

Minor edits clarification.

In reply to Re: How to test for empty hash? by haukex
in thread How to test for empty hash? by scareduck

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