when ($_ >= 8 && $_ <= 11)     # $index here is min( 2, 5), so 2
    when ($_ >= 12 && $_ <= 15) {  # $index here is min( 3, 5), so 3
    when ($_ >= 16 && $_ <= 19) {  # $index here is min( 4, 5), so 4
    when ($_ >= 20) {              # $index here is min( 5+,5), so always bounded at 5