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Perl has a large number of punctuation variables. They control the behavior of various operations in the Perl interpreter. While they are initially set to reasonable default values, any Perl code has the ability to change their values for its own internal purposes. If a program modifies one of these variables, it is obligated to reset the variable back to its default value, lest it alter the behavior of subsequent unrelated code. The easiest way for a program to 'clean up after itself' is to declare such variables local when modifying them.

Noncompliant Code Example

This noncompliant code example shows a subroutine that counts the number of virtual users on this platform. This is deduced by the number of users in the /etc/passwd file that use the program /usr/bin/false as their shell.

sub count_virtual_users {
  my $result = 0;
  $/ = ":";
  open( PASSWD, "<", "/etc/passwd");
  while (<PASSWD>) {
    @items = split "\n";
    foreach (@items) {
      if ($_ eq "/usr/bin/false") {
        $result++;
      }
    }
  }
  $result;
}

This program produces the correct result, however it leaves the $/ variable set to an unusual value (:). Subsequent reads of any file will use this character as the end-of-line delimiter, rather than the typical newline, which is the default value.

Compliant Solution

This compliant solution again produces the same result, but localizes the punctuation variable. Consequently, when the subroutine returns, the $/ variable is restored to its original value, and subsequent file reads behave as expected.

sub count_virtual_users {
  my $result = 0;
  local $/ = ":";
  open( PASSWD, "<", "/etc/passwd");
  while (<PASSWD>) {
    @items = split "\n";
    foreach (@items) {
      if ($_ eq "/usr/bin/false") {
        $result++;
      }
    }
  }
  $result;
}

Exceptions

DCL02-EX0: This rule does not apply to object methods. Object methods are easy for the parser to distinguish from builtin functions or keywords due to their distinct syntax.

Related Guidelines

CERT C++ Secure Coding Standard: DCL32-CPP. Do not declare or define a reserved identifier
CERT C Secure Coding Standard: DCL37-C. Do not declare or define a reserved identifier

Exceptions

Risk Assessment

Modifying punctuation variables without declaring them local can corrupt data and create unexpected program behavior.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

DCL02-PL

low

probable

medium

P2

L3

Automated Detection

Tool

Diagnostic

Perl::Critic

Variables::RequireLocalizedPunctuationVars

Bibliography

[CPAN] Elliot Shank, Perl-Critic-1.116 Variables::RequireLocalizedPunctuationVars
[Wall 2011] perlfunc, perlvar


      01. Declarations and Initialization      DCL32-PL. Every module must return a true value

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