More typing for you, less typing for the poor souls who are actually calling your function. Swings and roundabouts.

Frankly, all the arguments based on catching odd numbers of arguments ring a bit hollow. The most likely case for this kind of bug is where someone carelessly writes

in which case using an anonymous hashref will only catch the problem if @bar has an even number of elements. That's a pretty marginal gain.

If you actually want your function to be robust, you still have to validate the parameters, or you're asking for all kinds of interesting fun when @bar is three elements long and $bar[1] eq 'foo' (or any other valid parameter name). So you still have to do extra typing, and you still have to throw runtime errors. In this case, having the parameters as a flat list is actually helpful: it means you can detect and warn about duplicate keys, instead of having them silently swallowed by the conversion to a hash.

Or you could be lazy and not validate ... but if you're not worried about even-lengthed garbage inputs, why do you care about odd-lengthed garbage inputs?