As Jeffrey pointed out it means that you can't reliably parse perl code without executing it.
in reply to Re: Perl Cannot Be Parsed: A Formal Proof
in thread Perl Cannot Be Parsed: A Formal Proof
This means that things like static code analysis, code transformation and syntax hilighting will never be reliable.
This is a drawback indeed, but on the other hand it means that modules can extend Perl's syntax, and that other nifty stuff can be accomplished.
So I understand this node as a proof of a property that is seldom fully understood.