Try adding some error checking. Example

sub computeLengths {
   my $self = shift;

   if (exists $self->[$__ref]->{IPv4} || exists $self->[$__ref]->{IPv6
+}) {
      my $ip = $self->[$__ref]->{IPv4} || $self->[$__ref]->{IPv6};
      if (exists $self->[$__ref]->{TCP}) {
         my $tcp = $self->[$__ref]->{TCP};
         $tcp->computeLengths;
         $ip->computeLengths({
            payloadLength => $tcp->getLength + $tcp->getPayloadLength,
         });
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