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Dr. Ludwig Ott, in his Fundamentals of Catholic Dogma, a standard theological reference source, states (pp. 203-207) that Mary's virginity, before, during, and after the birth of Jesus is de fide dogma (the highest level). One can search the book to some extent and see these passages, via the Amazon Online Reader function. There is also an online summary of Catholic dogmas, drawn from this book, where one can see Mary's perpetual virginity listed among many other dogmas.
Ott in turn cites Denzinger's Sources of Catholic Dogma, which is the authoritative source on the topic. This work is available online. Here are the relevant passages:
ST. MARTIN I 649-653 (655)
THE LATERAN COUNCIL 649
(Against the Monothelites)
256 Can. 3. If anyone does not properly and truly confess in accord with the holy Fathers, that the holy Mother of God and ever Virgin and immaculate Mary in the earliest of the ages conceived of the Holy Spirit without seed, namely, God the Word Himself specifically and truly, who was born of God the Father before all ages, and that she incorruptibly bore [Him?], her virginity remaining indestructible even after His birth, let him be condemned [see n. 218].
The Trinity and the Incarnation (against the Unitarians) *
[From the ordinance of Paul IV, "Cum quorundam,"* Aug. 7, 1555]
993 Since the depravity and iniquity of certain men have reached such a point in our time that, of those who wander and deviate from the Catholic faith, very many indeed not only presume to profess different heresies but also to deny the foundations of the faith itself, and by their example lead many away to the destruction of their souls, we, in accord with our pastoral office and charity, desiring, in so far as we are able with God, to call such men away from so grave and destructive an error, and with paternal severity to warn the rest, lest they fall into such impiety, all and each who have hitherto asserted, claimed or believed that Almighty God was not three in persons and of an entirely uncomposed and undivided unity of substance and one single simple essence of divinity; or that our Lord is not true God of the same substance in every way with the Father and the Holy Spirit, or that He was not conceived of the Holy Spirit according to the flesh in the womb of the most blessed and ever Virgin Mary, but from the seed of Joseph just as the rest of men; or that the same Lord and our God, Jesus Christ, did not submit to the most cruel death of the Cross to redeem us from sins and from eternal death, and to reunite us with the Father unto eternal life; or that the same most blessed Virgin Mary was not the true mother of God, and did not always persist in the integrity of virginity, namely, before bringing forth, at bringing forth, and always after bringing forth, on the part of the omnipotent God the Father, and the Son, and the Holy Spirit, with apostolic authority we demand and advise, etc.
[see also Denzinger #13, 20, 86, 91, 113, 144, 201-202, 214, 218, 227, 255, 282, 290, 344, 429, 462, 708-710, 734-735, 994, 1314, 1462; cf. Catechism of the Catholic Church, #496-511]
It is commonly understood that there are "four Marian dogmas":
1) Divine Motherhood / "Mother of God" / Theotokos
2) Perpetual Virginity
3) Immaculate Conception
4) Bodily Assumption
A web page by Fr. Johann G. Roten, S.M., gives an excellent overview of the rationale for each. A related website considers this very question: Should the perpetual virginity of Mary be considered a dogma?
Because this is already accepted as a given, often, discussions of the possible dogmatic declaration regarding Mary as Co-Redemptrix and Mediatrix of All Graces, are referred to as the "Fifth Marian Dogma" (example one / example two / example three / example four).