The documentation of length answers your question:

'length Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of $_. Note that this cannot be used on an entire array or hash to find out how many elements these have. For that, use "scalar @array" and "scalar keys %hash" respectively.'

In reply to Re: Wrong result for "length keys %$hashref"? by philipbailey
in thread Wrong result for "length keys %$hashref"? by samwyse

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