generating an error within an action

I'm parsing a list of statements where names must be declared first before
being used. If I detect in an action that a statement uses a name that
hasn't been declared (because I have a hash of the declarations), how can
I cause PRD to report a meaningful error?  I know I can return undef from
the action to cause the parse to fail, but I'd like to add a helpful
message. I can't find this in the pod or faq.

Thanks, Dave

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