Programming Perl, 4th Edition

C-Style Logical (Short-Circuit) Operators

Like C, Perl provides the && (logical and) and || (logical or) operators. Perl also provides a variant of ||, the logical defined or operator, //. These evaluate from left to right (with && having slightly higher precedence than || or //), testing the truth of the statement. These operators, shown in Table 3-8, are known as short-circuit operators because they determine the truth of the statement by evaluating the fewest number of operands possible. For example, if the left operand of an && operator is false, the right operand is never evaluated because the result of the operator is false regardless of the value of the right operand.

Table 3-8. Logical operators

Example	Name	Result
$a && $b	and	$a if $a is false, $b otherwise
$a || $b	or	$a if $a is true, $b otherwise
$a // $b	Defined or	$a if $a is defined, $b otherwise
$a and $b	Low precedence and	$a if $a is false, $b otherwise
$a or $b	Low precedence or	$a if $a is true, $b otherwise
$a xor $b	Low precedence xor	True if exactly one of $a or $b is true, false otherwise

Such short circuits not only save time but are also frequently used to control the flow of evaluation. For example, an oft-appearing idiom in Perl programs is:

open(FILE, "<", "somefile") || die "Can't open somefile: $!\n";

In this case, Perl first evaluates the open function. If the value is true (because somefile was successfully opened), the execution of the die function is unnecessary, and so it is skipped. You can read this literally as “Open ...