strict

use strict;         # Install all three strictures.

use strict "vars";  # Variables must be predeclared
use strict "refs";  # Can't use symbolic references
use strict "subs";  # Bareword strings must be quoted

use strict;         # Install all...
no strict "vars";   # ...then renege on one

use v5.12;          # by default with v5.12.0 or later

This lexically scoped pragma changes some basic rules about what Perl considers to be legal code. Sometimes these restrictions seem too strict for casual programming, such as when you’re just trying to whip up a five-line filter program. The larger your program, the stricter you need to be about it. If you declare a minimum version of perl with use, and that minimum version is v5.12 or later, you get strictures implicitly.

Currently, there are three possible things to be strict about: subs, vars, and refs. If no import list is supplied, all three restrictions are assumed.

strict "refs"

This generates a runtime error if you try to dereference a string instead of a reference, whether intentionally or otherwise.

See Chapter 8 for more about these.

use strict "refs";

$ref = \$foo;       # Store "real" (hard) reference
print $$ref;        # Dereferencing is ok

$ref = "foo";       # Store name of global (package) variable
print $$ref;        # WRONG, runtime error under strict refs

Symbolic references are suspect for various reasons. It’s surprisingly easy for even well-meaning programmers to invoke them accidentally; strict "refs" guards against that. Unlike real references, symbolic references can ...

Get Programming Perl, 4th Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.