use Socket;

while ( <> ) {
    s{([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})}{
        $addr = inet_aton $1 or die "Invalid IP address\n";
        inet_ntoa pack 'N', unpack( 'N', $addr ) - 1;
        }e;
    print;
    }
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