Programming Perl, 4th Edition

defined

defined EXPR
defined

This function returns a Boolean value saying whether EXPR is a defined value. Most data you deal with is defined, but a scalar that contains no valid string, numeric, or reference value is said to contain the undefined value, or undef for short. Initializing a scalar variable to a particular value defines it, and it stays defined until you assign an undefined value to it or explicitly call the undef function on that variable.

Many operations return undef under exceptional conditions, such as at end-of-file, when using an uninitialized variable’s value, an operating system error, etc. Since undef is just one kind of false value, a simple Boolean test does not distinguish between undef, numeric zero, the null string, and the one-character string, “0”—all of which are equally false. The defined function lets you distinguish between an undefined null string and a defined null string when using operators that might return a real null string.

Here is a fragment that tests a scalar value from a hash:

print if defined $switch{D};

When used on a hash element like this, defined tells only whether the value is defined, not whether the key has an entry in the hash. It’s possible to have a key whose value is undefined; the key itself, however, still exists. Use exists to determine whether the hash key exists.

In the next example, we exploit the convention that some operations ...