Your workaround assumes that somehow is_nullary can determine, if perhaps only at run time, whether whatever is nullary or not.
But this is an entirely different problem. No one claims that calling halt() doesn't make any sense just because it is statically undecidable. This is true for arbitrary programs ( functions ) in a Turing complete language and no one claims that function composition is useless because we can never know whether all of them terminate.
The implicit assumption of your article is that the parser derives one and only one parse tree ( solves all ambiguities ) from an expression and only this assumption has to be given up. The assertion becomes much weaker then:
A Perl parser can't resolve all ambiguities of a Perl program by context sensitive analysis
and the solution to this to derive multiple parse trees for an expression and wrap them into a common node that can represent a conditional expression for example.
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