$_="FourThreeTwoOne, Three, Four, One, Two";
$1="Three";
$second="&&&&";
s/$2/$second/g;
print;

##</code><code>##

$1="Three";

##</code><code>##

s/$2/$second/g;

##</code><code>##

{
    local $_="FourThreeTwoOne, Three, Four, One, Two";
    # ...
}

##</code><code>##

use strict;
use warnings;

##</code><code>##

my $second="&&&&";