my %mthSO = do { my $order; map { $_, ++ $order } qw{ ... }; };
...
$mthSO{ $a->[ 1 ] } cmp $mthSO{ $b->[ 1 ] }

Since month names are map-ped to numeric values, shouldn't the month name value comparison be numeric  <=> rather than lexicographic? Adding the dates  Jan14 Dec14 to the test set shows the problem.