sub decode_submit_v4 {
    my $pdu = shift;
    ($pdu->{message_class},     # 1 (2)
     $pdu->{source_addr_ton},   # 2 (1)
     $pdu->{source_addr_npi},   # 3 (1)
     $pdu->{source_addr},       # 4 (n+1)
     ) = unpack 'nCCZ*', $pdu->{data};
    my $len = 2	+ 1 + 1 + length($pdu->{source_addr}) + 1;

    ($pdu->{number_of_dests}) = unpack 'N', substr($pdu->{data}, $len);
    $len += 4;
    #warn "a decode_submit $len ($pdu->{number_of_dests}): ".hexdump(substr($pdu->{data}, $len));
    ....