#this is assuming you are in the right directory
#this is not the most efficient way of getting a list of files
foreach (<*.*>) {
   my($month,$year)=split(/\d{2}/,$_);
   $year+=1900;
   #Assuming that year started after 1990
   if($year<90){$year+=100;}
   $NEWSLETTER{$year}{$month} = 0;
}
foreach $y (sort keys %NEWSLETTER) {
   foreach $m (sort keys $NEWSLETTER{$y}) {
      #insert code for what you want to do with the sorted year and month....
   }
}