Page 9 of De Lacy Bellengari's "The Roll of the House of Lacy" states "Robert's wife was Matilda Countess de Perche, but he offers no source and no other researcher has been able to identify her more specifically than merely "Matilda". I am told that ES shows no countess of Perche as marrying a deLacy.
"W E Wightman, "The Lacy Family in England and Normandy, 1066-1194".