my $error = 0;
my @gaps;
my @tmp = sort datesort qw( 2000-4 2000-12 2001-1 2002-12 1999-12 );
my($year,$month) = split '-', $tmp[0];
for(my $i = 1; $i < @tmp; $i++) {
   my $gap = 0;
   my @range;
   do {
     $range[1] = "$year-$month";
     if(++$month == 13) {
        $month = 1;
        $year++;
     }
     if($tmp[$i] ne "$year-$month") {
        $range[0] = "$year-$month" unless $gap;
        $gap++;
        $error = 1;
     }
     if($gap > 100) {
       print "ERROR! infinite loop\n";
       last;
     }
   } until($tmp[$i] eq "$year-$month");
   if($gap) {
     push @range, "$year-$month";
     push @gaps, \@range;
   }
}
if($error) {
  for(@gaps) {
    print "Gap from $_->[0] to $_->[1]\n";
  }
}
print "Success\n" unless $error;
sub datesort {
  my($a1,$a2) = split '-', $a;
  my($b1,$b2) = split '-', $b;
  $a1 <=> $b1 or $a2 <=> $b2;
}
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This stores the ranges...
                - Ant