I concur with your assessment.

• You've clearly followed the example.

• It doesn't appear to be an error in the documentation. It is clearly describing something which cannot be done without the code you specified.

• $args{C} can accept an arbitrary hash instead reference as easily as the default anonymous one.

I checked rt.cpan.org, and there's no bug report for it. This distribution's bug reports. U: So I've created one.

By the way, I'd use $args{C} = $args{HASH} || \%cache; in case $args{HASH} exists, but isn't defined.