use Data::Dump qw/dd/;
my $feed = ...; # parse the data
print dd($feed);

# this is a long shot, it will be clearer when you find what $feed contains
# it is based on https://github.com/OneSkyWalker/protobuf-java-examples/blob/master/ListBusInfo.java
# getEntityList() is from the java api, what's in Perl? Seave through the dd output above.
#foreach $entity ($feed->getEntityList){
#  print dd($entity);
#  my $veh = $entity->vehicle;
#  my $position = $veh->position;
#  print "bus is at " . $position->{latitude} . ', ' .  $position->{longitude};
#}
# or
$entity = $feed->EntitySelector("42558_202397_3000_41_1");
print dd($entity);