DCL33-PL. Declare identifiers before using them

Perl provides the my() and our() functions specifically for declaring variables.

However, Perl allows any variable to be referenced, even if it is not declared or initialized. If an uninitialized value is requested, Perl supplies a default undef value. Depending on the context, the undef value may be interpreted as 0, false , or an empty string.

Because Perl programs are typically not explicitly compiled before they are run, they can suffer from typographic errors in variable names. A variable whose name is typed incorrectly will appear as an undeclared variable to the Perl interpreter and consequently will contain the undef value instead of the value of the intended variable.

Because of the hazard of mistyped variables, all variables should be declared before use.

Noncompliant Code Example

This noncompliant code example contains a typo in its print statement.

my $result = compute_number();
print "The result is $reuslt\n";   # oops!

It causes the program to print the following useless output:

The result is

and continue execution.

Compliant Solution

This compliant solution corrects the typo, causing the program to correctly print the result of compute_number() .

my $result = compute_number();
print "The result is $result\n";

Risk Assessment

Using undeclared variables usually can lead to incorrect results and surprising program behavior.

Rule	Severity	Likelihood	Detectable	Repairable	Priority	Level
DCL33-PL	Low	Probable	Yes	Yes	P6	L2

Automated Detection

Tool	Diagnostic
`use warnings;`	Name .* used only once. possible typo
`use strict;`	Global symbol .* requires explicit package name
Perl::Critic	Policy::TestingAndDebugging::RequireUseWarnings
	Policy::TestingAndDebugging::RequireUseStrict

SEI CERT C Coding Standard

DCL31-C. Declare identifiers before using them

Bibliography