You're right; I've since realized that what this code should do is

if (my @bad = grep { /^(dualvar|set_prototype)$/ } @_ ) { ... }
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And then give @bad in the error message. (This'll give a bogusly redundant list if the import spec of Scalar::Util contains bogus redundancies, but I doubt that's a problem.)

Anyway there's now a bug in the RT queue for this distro with the correct fix in it.