based on this from Google::ProtocolBuffers's synopsis:

##
## Decode data from serialized form
##
my $person;
{
    open my($fh), "<person.dat";
    binmode $fh;
    local $/;
    $person = Person->decode(<$fh>);
    close $fh;
}
print $person->{name}, "\n";
print $person->name,   "\n";  ## ditto
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you should somehow end up with an $entities object. Then use dd to dump it and see if you can find about its methods or stored hashkeys.

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 con
+tains
# it is based on https://github.com/OneSkyWalker/protobuf-java-example
+s/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->{lo
+ngitude};
#}
# or
$entity = $feed->EntitySelector("42558_202397_3000_41_1");
print dd($entity);
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