Which of those is true?

Both, they're not mutually exclusive :) if you keep reading it also says

The behavior of kill when a *PROCESS* number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group and -1 will signal all processes.

See also what perlport#kill says

In reply to Re^4: Vague segfault question by Anonymous Monk
in thread open3() problem by ChrisDennis

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