SO_REUSEPORT is Linux-specific option to facilitate multi-threaded servers:
SO_REUSEPORT (since Linux 3.9)
Permits multiple AF_INET or AF_INET6 sockets to be bound to an identical socket address.
For TCP sockets, this option allows accept(2) load distribution in a multi-threaded server to be improved by using a distinct listener socket for each thread. This provides improved load distribution as compared to traditional techniques such using a single accept(2)ing thread that distributes connections, or having multiple threads that compete to accept(2) from the same socket.
Probably shouldn't be relevant here.

In reply to Re^2: IO::Socket persisting on Linux despite being closed by Anonymous Monk
in thread IO::Socket persisting on Linux despite being closed (updated) by davies

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