sub nextmonth {
           my($year, $month);
           $year = substr($_[0], 0, 4);
           $month = substr($_[0], 4, 2);
        
           $month++;
         
           while ($month > 12) {
               $year++;
               $month-=12;
           }
           
           return $year . sprintf("%02d", $month);
            
       }

       sub find_gaps {
           my $dates = shift;
           my(@missing, @keys, $last);

           # convert your date-format to mine
           @keys = map {  s/-(.)$/0$1/; s/-//; $_ } keys %$dates;

           # my format can be sorted numerically
           @keys = sort @keys;
           
           # in the loop we need two variables:
           # $last for the date before the current one,
           # and $date for the current date

           $last = shift(@keys);
           foreach my $date ( @keys ) {
                while (1) {
                   $last = nextmonth($last);
                   last if $last eq $date;
                   push(@missing, $last);
               }
               $last = $date; 
           } 

           # convert my date-format back to yours
           @missing = 
               map {  s/0(.)$/$1/; s/^(....)/$1-/; $_ }  @missing;
           return \@missing;
       }