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In list context, things are slightly more complicated. An empty list, when evaluated in a Boolean boolean condition, evaluates to false. But the value undef, when evaluated in list context, evaluates to true because it is converted to a list with the singleton value undef. Therefore, a function should not return undef if it might ever be invoked in list context.
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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 class condition returns true, and the system incorrectly prints out:
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