避免使用 '$1' 和 '$2'，因为它们是正则表达式元字符.

Avoid '$1' and '$2' because they are regex metacharacters.

use strict;
use warnings;

my $val1 = "abc";
my $val2 = "def";
while (<>)
{
    s/$val1/$val2/g;
    print;
}