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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.

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This noncompliant code example shows a subroutine that counts the number of virtual users on this platform. This is value 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 but 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.

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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;
}

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DCL02-EX0: The following global variables may be modified without being declared local:

Risk Assessment

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

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