DCL31-PL. Do not overload reserved keywords or subroutines

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 codes a subroutine called open(), which clashes with the open() built-in function.

sub open {
  my ($arg1, $arg2, $arg3) = @_;
  print "arg1 = $arg1\n";
  print "arg2 = $arg2\n";
  print "arg3 = $arg3\n";
}

open( my $input, "<", "foo.txt");     # What does this do?

Perl (v5.12.1) actually invokes the built-in open() rather than the newly crafted subroutine.

Compliant Solution

This compliant solution uses a different name for its subroutine; consequently, it behaves as expected.

sub my_open {
  my ($arg1, $arg2, $arg3) = @_;
  print "arg1 = $arg1\n";
  print "arg2 = $arg2\n";
  print "arg3 = $arg3\n";
}

my_open( my $input, "<", "foo.txt");

Exceptions

DCL31-EX0: This rule does not apply to object methods. Object methods are easy for the parser to distinguish from built-in 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

Risk Assessment

Using reserved keywords can lead to unexpected program behavior and surprising results.

Automated Detection

Bibliography

[Conway 05] pg. 177 "Homonyms"

[CPAN] Elliot Shank, Perl-Critic-1.116 Subroutines::ProhibitBuiltinHomonyms
[Wall 2011] perlfunc, perlsyn