In particular, the Methodology section makes it clear that some tests are deliberately written non-idiomatically so that they can test common features of all the languages.

This notion of non-idiomatic should be clearly defined, to my mind,
it's (at best) unfair, beccause it gives a serious advantage to the ones considered as 'idiomatic' (C?).
Moreover why compare languages if you can't use their own strengths ?

Anyway I don't think that adding useless instruction in Perl make it 'non-idiomatic Perl'.

Furthermore look at other language examples I DO find that some of them use language specific idioms.
(some haskell and ruby code for example)

UPDATE :

From the methodology text :

Since functional languages have such a different mode of expression, I allow them more leeway

In some cases I'm not really measuring the speed of a language, but how good of a programmer I am in that given language.

"Only Bad Coders Code Badly In Perl" (OBC2BIP)

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