For example, I want to split "one,two,three" with comma as delimiter and use a loop to process the resulted three substring separately.
Answer
A simpler solution than the current one presented involves using the built-in substitution modifer -- there is no need or reason to wastefully use a loop or external command substitution in this instance:
set list = one,two,three
set split = ($list:as/,/ /)
echo $split[2] # returns two
() creates a list, the :s is the substitution modifier and :as repeats the subtitution as many times as needed.
Furthermore, t/csh does not require quoting of bare strings, nor variables that do not require forced evaluation.