those who agree with the statement could produce a piece of functionality in another language they think cannot be done in perl.

But that's not enough. You said that "(Perl) really is the only language that can do everything any other language can do.". To prove that, you need to prove that no other language can do everything any other language can do. So for each other language l, other than Perl, you need to find something that l cannot do.

That's the part of your statement that I thought was most mistaken, anyway.