my %month_key_lookup = (
    'Jan' => 'a',
    'Feb' => 'b',
    ...
    'Dec' => 'l',
);

my @sorted_dates =
    map substr($_, 12),
    sort
    map { /^(.{3}) (\d?)(. .*)/s;
          $month_key_lookup{$1}.($2||'0').$3.$_ }
    @dates;