the extra \& noise isn't going to prevent it from blowing up at run-time

The whole point of parameter validation is to blow up. Why would you want to prevent it?

Without the "extra noise", it wouldn't blow up. That's the problem.

sub f0    { my ($cb) = @_; '...' }
sub f1(&) { my ($cb) = @_; '...' }

f0(\&abc);                  # OK No error
f0(\&{ maybe_coderef() });  # OK Error caught at run-time (if any)
f0("abc");                  # XX Error uncaught
f0(maybe_coderef());        # XX Error uncaught (if any)

f1(\&abc);                  # OK No error
f1(\&{ maybe_coderef() });  # OK Error caught at run-time (if any)
f1("abc");                  # OK Error caught at compile-time
f1(maybe_coderef());        # OK Error caught at compile-time (if any)
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Update: Added example