Programming Perl, 4th Edition

Assignment Operators

Although it’s not exactly a mathematical operator, we’ve already made extensive use of the simple assignment operator, =   . Try to remember that = means “gets set to” rather than “equals”. (There is also a mathematical equality operator == that means “equals”, and if you start out thinking about the difference between them now, you’ll save yourself a lot of headache later. The == operator is like a function that returns a Boolean value, while = is more like a procedure that is evaluated for the side effect of modifying a variable.)

Like the operators described earlier, assignment operators are binary infix operators, which means they have an operand on either side of the operator. The right operand can be any expression you like, but the left operand must be a valid lvalue (which, when translated to English, means a valid storage location like a variable, or a location in an array). The most common assignment operator is simple assignment. It determines the value of the expression on its right side, and then sets the variable on the left side to that value:

$a = $b;
$a = $b + 5;
$a = $a * 3;

Notice the last assignment refers to the same variable twice; once for the computation, once for the assignment. There’s nothing wrong with that, but it’s a common enough operation that there’s a shortcut for it (borrowed from C). If you say:

lvalue operator= expression

it is evaluated as if it were:

lvalue = lvalue operator expression

except that the lvalue is not computed twice. ...