There are, as already mentioned, several issues with the index you name. Even so, Perl only slipped 3 places by their count and is still at 5 times the threshold they use to determine "mainstream programming languages". They say "it is strongly advised to stick to mainstream languages for industrial, mission-critical software systems" on the page defining the index.
in reply to Perl is sinking (TIOBE): all time low for Perl
Even if this is taken as a valid metric, it is only claimed to be one metric. The people who publish it specifically state it does not represent the best programming language or the amount of lines written in any particular language. Any intelligent discussion of a language takes far more into account than one metric.