Programming Perl, 4th Edition

Controlling the Use of Globals

Another commonly seen declaration is the strict pragma, which has several functions, one of which is to control the use of global variables. Normally, Perl lets you create new globals (or, all too often, step on old globals) just by mentioning them. No variable declarations are necessary—by default, that is. Because unbridled use of globals can make large programs or modules painful to maintain, you may sometimes wish to discourage their accidental use. As an aid to preventing such accidents, you can say:

use v5.14;            # Turn on strict implicitly.
use strict "vars";    # Turn on strict explicitly.

This means that any variable mentioned from here to the end of the enclosing scope must refer either to a lexical variable declared with my, state, or our, or to an explicitly allowed global. If it’s not one of those, a compilation error results. A global is explicitly allowed if one of the following is true:

• It’s one of Perl’s program-wide special variables (see Chapter 25).

• It’s fully qualified with its package name (see Chapter 10).

• It’s imported into the current package (see Chapter 11).

• It’s masquerading as a lexically scoped variable via an our declaration. (This is the main reason we added our declarations to Perl.)

Of course, there’s always the fifth alternative—if the pragma proves burdensome, simply countermand it within an inner block using:

no strict "vars";

You can also turn on strict checking of symbolic dereferences and accidental use of barewords with this ...