如何使用perl替换字符串

#! perl
 
$string0 = "Hello world";
$string1 = "world";
$string2 = "onitroad";
$caseon  = "";
$caseoff = "";

#如果不区分大小写，使用下面开关
$caseon  = "(?i)";
$caseoff = "(?-i)";
 
# 使用正则表达式查找并替换
$string0 =~ s/$caseon$string1$caseoff/$string2/g;
 
print "$string0\n";