Perl in a Nutshell, 2nd Edition

Scalar and List Contexts

Every operation that you invoke in a Perl script is evaluated in a specific context, and how that operation behaves may depend on the context it is being called in. There are two major contexts: scalar and list. All operators know which context they are in, and some return lists in contexts wanting a list and scalars in contexts wanting a scalar. For example, the localtime function returns a nine-element list in list context:

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(  );

But in a scalar context, localtime returns the number of seconds since January 1, 1970:

$now = localtime(  );

Statements that look confusing are easy to evaluate by identifying the proper context. For example, assigning what is commonly a list literal to a scalar variable:

$a = (2, 4, 6, 8);

gives $a the value 8. The context forces the right side to evaluate to a scalar, and the action of the comma operator in the expression (in the scalar context) returns the value farthest to the right.

Another type of statement that might be confusing is the evaluation of an array or hash variable as a scalar. For example:

$b = @c;

When an array variable is evaluated as a scalar, the number of elements in the array is returned. This type of evaluation is useful for finding the number of elements in an array. The special $# array form of an array value returns the index of the last member of the list (one less than the number of elements).

If necessary, you can force a scalar context in ...