my $monthConverter = monthConverterFactory();

...

my $moNum = &$monthConverter($mo);
   if ($moNum > $currentDate[1]) {
   $y = $currentDate[2] - 1;
}

...

# Use a closure here, so we don't have to load this private
# data more than once.
sub monthConverterFactory {
   my %months = ( Jan => 0,
      Feb => 1,
      Mar => 2,
      Apr => 3,
      May => 4,
      Jun => 5,
      Jul => 6,
      Aug => 7,
      Sep => 8,
      Oct => 9,
      Nov => 10,
      Dec => 11 
   );
   return sub {
      my $val = shift;
      return $months{$val};
   }
}