s/           # begin substitution
   ([%\d])   # A captured character class for
             #  a literal "%" and then \d
             #  -- does that even work?
             #  I would write it as [%0-9], but ok,
             #  that's so beside the point.

   $_        # Ok, so we match $_ *AFTER* we matched
             # that character class

   ([%\d])   # And there's that cutie again