I'd go for something like:

  my @s = ();
  local $" = '.';
  @out = map { $#s=--$_; ++$s[$_]; "@s" } @a;
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except that I'd do it in a way that didn't modify the source array. Of course, if this were homework it would probably be better to write it in a way that showed I understood what I was doing. :)

Hugo