Ruby 3.3 リファレンスマニュアル

variable $FIELD_SEPARATOR

$FS -> String | nil[permalink][rdoc][edit] [added by English]
$FIELD_SEPARATOR -> String | nil [added by English]

$; の別名 

require "English"

str = "hoge,fuga,ugo,bar,foo"
p str.split # => ["hoge,fuga,ugo,bar,foo"]
$FIELD_SEPARATOR = ","
p str.split # => ["hoge", "fuga", "ugo", "bar", "foo"]