I call mine Abusing substr. The idea: convert it to zero padded month and then use substr to walk the date range from the first to the last, pushing onto the array ones you don't have. I hate having to convert it back to YYYY-M+ for testing.

I am absolutely baffled by MeowChow solutions. What the heck does +pop do?

#!/usr/bin/perl -w 
 
use strict; 
 
sub find_gaps { 
  my $dates=shift; 
  my @dates=sort map { sprintf "%04d-%02d",split/-/ } keys %$dates; 
  my ($c,$l)=($dates[0],$dates[$#dates]); 
  my @gaps; 
 
  while ($l cmp $c) { 
    my $r=sprintf("%d-%d",split(/-/,$c)); 
    push @gaps,$r unless defined $dates->{$r}; 
    substr($c,5,2)++; 
    if (substr($c,5,2)==13) { 
      substr($c,5,2)='01'; 
      substr($c,0,4)++; 
    } 
  } 
 
  \@gaps; 
} 
 
 
 
my %dates=map { $_,'' } qw ( 2000-4 2000-2 2000-1 2000-12 2001-1 2002-
+12 1999-12 ); 
my $gaps=find_gaps \%dates; 
 
print "dates\n",join("\n",sort keys %dates),"\n"; 
print "gaps\n",join("\n",@$gaps),"\n";
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