Programming Perl, 3rd Edition

Assignment Operators

Perl recognizes the C assignment operators, as well as providing some of its own. There are quite a few of them:

=    **=    +=    *=    &=    <<=    &&=
            -=    /=    |=    >>=    ||=
            .=    %=    ^=
                  x=

Each operator requires a target lvalue (typically a variable or array element) on the left side and an expression on the right side. For the simple assignment operator:

TARGET = EXPR

the value of the EXPR is stored into the variable or location designated by TARGET. For the other operators, Perl evaluates the expression:

TARGET OP= EXPR

as if it were written:

TARGET = TARGET OP EXPR

That's a handy mental rule, but it's misleading in two ways. First, assignment operators always parse at the precedence level of ordinary assignment, regardless of the precedence that OP would have by itself. Second, TARGET is evaluated only once. Usually that doesn't matter unless there are side effects, such as an autoincrement:

$var[$a++] += $value;               # $a is incremented once
$var[$a++] = $var[$a++] + $value;   # $a is incremented twice

Unlike in C, the assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable to which it was assigned. This is useful for modifying a copy of something, like this:

($tmp = $global) += $constant;

which is the equivalent of:

$tmp = $global + $constant;

Likewise:

($a += 2) *= 3;

is equivalent to:

$a += 2;
$a *= 3;

That's not terribly useful, but here's an idiom you see frequently:

($new = $old) =~ s/foo/bar/g;

In all ...