switch文を使うと見やすいんじゃないかな?

use Switch

switch ($MON) {
case [1,3,5,7,8,10,12] { $m = 31 }
case [4,6,9,11] { $m = 30 }
case [2] { $m = $YEAR % 4 ? 28 : 29 }
else { print "そんな月知らん" }
}