My main objection to the use of plurals almost anywhere in software, is that it usually leads to wasted time having to look up whether the plural or singular was used this time (because it is easy to remember the name but very easy to forget whether the name was plural or not). But variables usually have very limited scope, so that means that this problem doesn't apply for most variables.
And I am at least somewhat reluctant to give two different variables the same name in overlapping scopes, so writing for my $friend ( @friend ) gives me pause. I've certainly done it from time to time. But I've also often enough regretted it. It is a pain to not be able to, for example, rename one of the variables without having your search also stop on all the uses of the other variable (say when you change from an array to a hash).
I also find it more reasonable to reconcile $fiends over @friend, since the "one of" modifier (like ) get placed right there next to the pluralizing letter(s). :)