Rather than adding an additional triple pattern you actually want to use a FILTER
with the ISURI
function e.g.,
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
PREFIX dbpprop: <http://dbpedia.org/property/>
SELECT ?philosopher ?era
WHERE {
?philosopher a dbpedia-owl:Philosopher ;
dbpprop:era ?era .
FILTER(ISURI(?era))
}