Are trailing commas in Perl a bad practice?

The Apache mod_perl coding style document says this

Whenever you create a list or an array, always add a comma after the last item. The reason for doing this is that it's highly probable that new items will be appended to the end of the list in the future. If the comma is missing and this isn't noticed, there will be an error.

What your manager may have been thinking of is that doing the same thing in C is non-standard and non-portable, however there is no excuse for his extraordinary behaviour.

It's a great practice to have the trailing comma. Larry added it because he saw programmers add elements to a list (or whatever their language called it) but forget the separator character. Perl allows the trailing comma to make that less common. It's not a quirk or side effect of something else. That's what Perl wants you to do.

What is bad practice, however, is distracting a meeting full of people with something your boss could have corrected later. Unless the meeting was specifically a code review, your boss wasted a bunch of time. I've always wished that to join a video conference, you had to enter your per-minute compensation so a counter would show on everyone's screen to show how much money was being wasted. Spending a couple hundred dollars watching you remove commas on a working program would tamp down that nonsense.

So the PBP page referred to by Miller argues for making it easier to reorder the list by cutting and pasting lines; the mod_perl coding style document linked by Borodin argues for avoiding a momentary syntax error when you add stuff.

Much more significant than either, in my opinion, is that if you always have a trailing comma and you add a line, the diff only shows the line you added and the existing lines remain unchanged. This makes blame-finding better, and makes diffs more readable.

All three are good reasons for always using trailing commas, and there are in my opinion no good reasons not to do so.