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

In reply to Re: Finding gaps in date ranges by MeowChow
in thread Finding gaps in date ranges by Ovid

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