月名を月に変換

$mon = {
    Jan => 0,
    Feb => 1,
    Mar => 2,
    Apr => 3,
    May => 4,
    Jun => 5,
    Jul => 6,
    Aug => 7,
    Sep => 8,
    Oct => 9,
    Nov => 10,
    Dec => 11,
}->{$mon};

↓

$mon = (index 'JanFebMarAprMayJunJulAugSepOctNovDec', $mon) / 3;