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3.17. 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 cases, the value of the assignment is the new value of the
variable. Since assignment operators associate right-to-left, this can
be used to assign many variables the same value, as in:
$a = $b = $c = 0;
which assigns 0 to $c, and the result of that (still 0) to
$b, and the result of that (still0) to $a.
List assignment may be done only with the plain assignment operator,
=. In list context, list assignment returns the list of new values
just as scalar assignment does. In scalar context, list assignment
returns the number of values that were available on the right side of
the assignment, as mentioned in Chapter 2, "Bits and Pieces".
This makes it useful for testing functions that return a null list when
unsuccessful (or no longer successful), as in:
while (($key, $value) = each %gloss) { ... }
next unless ($dev, $ino, $mode) = stat $file;