The documentation is wrong.

While the point another commenter made about can and isa being overridden is valid, that doesn't mean that you should never use them like that. Just that you should be really sure that that's what you want to do.

In reply to Re: Do not use UNIVERSAL::isa this way; but why? by DrHyde
in thread Do not use UNIVERSAL::isa this way; but why? by klekker

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