I would approach this problem with a conversion, from date format, to a more easily dealt-with numeric format, and vice-versa:

sub find_gaps {
  my ($pdate, @dates) = sort {$a <=> $b} 
                        map  { my ($y,$m) = split /-/; $y*12 + $m - 1 
+} 
                        keys %{+pop};
  my @skips;
  foreach (@dates) {
    push @skips, $pdate+1 .. $_-1;
    $pdate = $_;
  }
  [ map { int ($_/12) . "-" . (1 + $_%12) } @skips ];
}
[download]

UPDATE: couldn't help golfing things a bit...

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print