What leads you to believe that the people who believe things like TIOBE are interested in measurable facts instead of feel-good woo?
Well I think because people who think TIOBE is right are in large numbers. Now regardless what TIOBE actually is, that's irrelevant to what it is perceived by majority of the programmers/business guys alike
When they say something is wrong with Perl or that its usage is going down, people believe them. You may not agree, but when such large number of people believe in it, the damage is already done.