DCL04-PL. Always initialize local variables

Perl has a large number of built-in functions; they are described on the perlfunc manpage [Wall 2011]. Perl also has a handful of reserved keywords such as while; they are described on the perlsyn manpage [Wall 2011].

Do not use an identifier for a subroutine that has been reserved for a built-in function or keyword.

Noncompliant Code Example

This noncompliant code example authenticates the user to enter a password, but only if the $passwd_required variable is defined.

$passwd_required = 1;

# ...

sub authenticate_user {
  local $passwd_required;
  if (defined $passwd_required) {
    print "Please enter a password\n";
    # ... get and validate password
  } else {
    print "No password necessary\n";
  }
}

authenticate_user();

The call to local temporarily sets $passwd_required to the uninitialized value undef; it does not maintain its previous value of 1. Consequently, when the program executes, it incorrectly prints No password necessary.

Compliant Solution

This compliant solution initializes the localized variable to the old value, so it correctly prompts the user for a password.

$passwd_required = 1;

# ...

sub authenticate_user {
  local $passwd_required = $passwd_required;
  if (defined $passwd_required) {
    print "Please enter a password\n";
    # ... get and validate password
  } else {
    print "No password necessary\n";
  }
}

authenticate_user();

Risk Assessment

Uninitialized variables can cause surprising program behavior.

Automated Detection

Bibliography

[Conway 05] pg. 78 "Initialization"
[CPAN] Elliot Shank, Perl-Critic-1.116 Variables::RequireInitializationForLocalVars
[Wall 2011] perlfunc, perlsyn