The entire point of the argument is that perl doesn't first completely parse the program before it runs it. When executing a Perl program perl alternates between parsing and running. That's what BEGIN is about (and hence, use).
in reply to Re: Perl Cannot Be Parsed: A Formal Proof
in thread Perl Cannot Be Parsed: A Formal Proof
The article proves that it's actually impossible to do otherwise. That is, there does not (and will not) exist a program that first completely parses a Perl program, then executes it. (Well, at least not as long as Perl remains as it is).