It is also written in perldoc perlipc:

Handling the SIGHUP Signal in Daemons
...
Not all platforms automatically reinstall their (native) signal handlers after a signal delivery.
This means that the handler works only the first time the signal is sent.
The solution to this problem is to use POSIX signal handlers if available, their behaviour is well-defined.

In reply to Re^3: SIGHUP handler "IGNORE" not defined. by sh1tn
in thread SIGHUP handler "IGNORE" not defined. by scottb

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