OK, it took me a few minutes to completely understand how and why this works. Here is my dissected version of the regex:

     s/(\d+)            # first number (group #1)
        (?:             # group #2
         ,              #   followed by a comma
         (              #   group #3
          (??{$++1})    #     match previous number + 1 (group 4)
         )              #   end group #3
        )+              # end group #4, repeat
       /$1-$+/gx;       # substitute for the first number followed by 
+the
                        # last matched one
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Group #1 matches the first number in a sequence of numbers. Then, the ??{$+ + 1} is used to match "the last number plus one" ($+ stands for whatever was matched by the last set of grouping parenthesis). For the second number in a sequence, the "last number" is the one matched by group #1. But for subsequent numbers (because of the +), the last number matched (this is, whatever the ??{$++1} matched last time) becomes the "last number". So the thing repeats until the "last number plus one" part doesn't match anymore (this is, until a non-consecutive number is found), and then replaces the whole thing with the first number (group #1), a dash, and the last number matched.

At first look, I thought the double parenthesis around ??{$++1} were unnecessary, but without them it does not work, and here is why: $+ contains what was matched by the last set of parenthesis, not the current set. So by doubling the parenthesis, it makes $+ contain the last thing matched by the current expression. Very clever!

--ZZamboni