Here is my code for this part of the initialization of pcap:

my %pcap_parameters = (
        SNAPLEN => 124,         # Num bytes to capture from packet
    PROMISCUOUS_MODE => 1,  # Operate in promiscuous mode?
        TIMEOUT => 1000,        # Read timeout (ms)
        NUMPACKETS => 500,       # Pkts to read (-1 = loop forever)
        #FILTER => 'ip proto \icmp',           # Filter string
        FILTER => 'udp dst port 161',           # Filter string
    USERDATA => '',         # Passed as first arg to callback fn
    SAVEFILE => '',         # Default save file

        # Items below are RETURNED values from PCap calls.
        # Do not attempt to change them in the declaration.
        FILTER_HANDLE => 0,     # Reference to compiled filter
        NETWORK_INTERFACE => '',# Network interface to open
        NETWORK_ADDR =>0,       # Network Address (32 bit number)
        NETWORK_MASK =>0,       # Mask (32-bit number)
    mode => '',             # Internal variable
        );
...
...
 #Get the name of a network device that can be used
  $pcap_parameters{NETWORK_INTERFACE} = Net::Pcap::lookupdev(\$err) or
+ die "No Network device found:$err\n";
  $verbose and print "Found device \t$pcap_parameters{NETWORK_INTERFAC
+E}\n";
  $result = Net::Pcap::lookupnet($pcap_parameters{NETWORK_INTERFACE},
              \$pcap_parameters{NETWORK_ADDR},
              \$pcap_parameters{NETWORK_MASK}, \$err);
  $verbose and print "Found Net \tnet " . NetPacket::IP::to_dotquad($p
+cap_parameters{NETWORK_ADDR}) .
        "  mask " . NetPacket::IP::to_dotquad($pcap_parameters{NETWORK
+_MASK}) . "\n";

  $pcap_desc = Net::Pcap::open_live($pcap_parameters{NETWORK_INTERFACE
+}, $pcap_parameters{SNAPLEN},
                        $pcap_parameters{PROMISCUOUS_MODE},
                        $pcap_parameters{TIMEOUT},
                        \$err)
         or  die("Net::Pcap::open_live returned error $err\n");
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