$sock isn't connected to anything. That's why you're getting 

Use of uninitialized value in subroutine entry
Bad arg length for Socket::inet_ntoa, length is 0, should be 4

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$! contains the following, but you might not have known that peeraddr sets it on error. 

Transport endpoint is not connected

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You want to know to whom $new_sock is connected.

In reply to Re^5: IO Socket - Detect inbound IP by ikegami
in thread IO Socket - Detect inbound IP by sans-clue

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