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
(still 0
) to
$a
.
List assignment may be done only with the plain
assignment operator, =
. In list context, list
assignment returns the list of new values just as scalar assignment
does. In scalar context, list assignment returns the number of values
that were available on the right side of the assignment, as mentioned
in Chapter 2. This makes it useful
for testing functions that return a null list when unsuccessful (or no
longer successful), as in:
while (($key, $value) = each %gloss) { … } next unless ($dev, $ino, $mode) = stat $file;