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Returning the value undef is a common convention for a function to indicate it has no return value. This It is often used to indicate that an error occurred or that a function could not successfully complete an operation. When used as the conditional in a conditional expression (such as in an if statement), undef evaluates to false. Therefore, a function that is evaluated only in scalar context may safely return undef to indicate failure.

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sub read_users {
  open( my $filehandle, "<", "/etc/shadow")
    or return undef;
  my @users = <$filehandle>;
  return @users;
}

# ...

if (my @users = read_users($filename)) {
  print "Your system has $#users users\n";
  # process users
} else {
  croak "Cannot read shadow file";
}

The read_users() subroutine returns undef if it cannot open /etc/shadow, but it returns a list of user data entries if it succeeds. Because its output is used in list context, a return value of undef is converted to a list of a single element: (undef). Consequently, the if condition returns true, and the system incorrectly prints out the following:

Your system has 0 users

Compliant Solution

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sub read_users {
  open( my $filehandle, "<", "/etc/shadow")
    or return;
  my @users = <$filehandle>;
  return @users;
}

Exceptions

EXP00-PL-EX1: This recommendation applies specifically to functions called in a list context. If you can guarantee that some function will never be called in a list context, then that function may return undef.

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Bibliography

[Conway 2005] pg. 199 

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