The former Estudiantes man, 28, has had a year of near misses, playing in losing World Cup and UEFA Europa League finals - although he did win the Primeira Liga title in Portugal.
The midfielder will enter 2015 in La Liga - his first stint in Spain - despite being linked with Premier League powerhouse Manchester United, and his move to the Mestalla will be confirmed on Friday pending a medical.
"Valencia Club de Futbol and SL Benfica have reached an agreement for the transfer of Enzo Nicolas Perez, on a permanent deal that will be made official on January 2nd, 2015," a club statement read.
"The move is subject to the player passing a medical.
"The Argentinian international will arrive at Manises-Valencia airport at 11.25pm local time and will undergo a medical tomorrow under the supervision of the Valencia Club de Futbol medical team."
Perez has appeared 11 times this league season for Benfica, scoring once - having scored four times in 28 appearances in each of his past two terms in Portugal.
