I misread the OP. The below can be ignored as it doesn't equate to the actual problem.

You just want to add your own custom fail report? Here's what I do at work:

    throws_ok
        { $Keys->add_domain_mapping }
        qr/requires a.*domain/,
        "add_domain_mapping() croaks if no domain sent in ok";

    throws_ok
        { $Keys->add_domain_mapping('blah.com') }
        qr/requires a.*selector/,
        "add_domain_mapping() croaks if no selector sent in ok";
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