Doesn't defined $config{ $_ } && $config{ $_ } have the same truth value as $config{ $_ }?

Yes. I was attempting to capture your stated requirements:

but omits them if their configured value is false, undefined, or nonexistent.

and got it wrong. It should be:

exists $config{ $_ } && $config{ $_ }

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In reply to Re^3: Taking a subset of a hash by BrowserUk
in thread [Answered!] Taking a subset of a hash by Narveson

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