According to Catholicism, did St. Paul believe that end of the world was imminent?

The Greek Fathers generally thought St. Paul was referring to the immanency of the General Judgment, and the Latins generally thought he was referring to one's particular judgment; these two senses are intended, with the immanency of the particular judgment being principal, according to Allo, O.P., Saint Paul: Première épître aux Corinthiens, p. 180, PDF p. 294.

St. Paul means that we do not have much time to work out our salvation (cf. Phil. 2:12: "with fear and trembling work out your salvation.") and must practice detachment from transient, worldly things.

Cornelius à Lapide on 1 Cor. 7:29:

Ver. 29.—But this I say, brethren, the time [καιρός] is short [contracted, συνεσταλμένος]. The duration of this life is short, so that we may not think of merely enjoying our wives and the things of this present life, but, as strangers and sojourners, use them for a short time, in order to travel better towards that glorious City into which we shall be enrolled as everlasting citizens. Ambrose takes the time here in a wider sense, as denoting the duration of the world. Time is short, and the day of judgment is at hand: do not, therefore, spend your time on the temporal pleasures of the world, but prepare yourselves for judgment.

St. John Chrysostom, On Holy Virginity §73 [PDF pp. 212-15]:

LXXIII. The present time is not for marriage.

1. Perhaps you will ask what has this [1 Cor. 7:29] to do with marriage? It is very relevant. If marriage has been con­fined to the present life and in the future people neither will marry nor be given in marriage [cf. Mt. 22:30], if the present speeds to its end and the resurrection stands at our door, it is not the time for marriage and possessions but for poverty and every other kind of wisdom of use to us in the next world. The young girl, so long as she remains at home with her mother, is occupied with childish cares. She deposits her little chest in the household treasury with its store; the key and all authority are in her possession. She is as solicitous of those little trifles as guardians of important households are of what is in their charge. However, when she must be betrothed and marriage forces her to leave her father's house, then she is removed from that world of trifles and her lowly position. She is compelled to manage the household with its many possessions and slaves, to tend to her husband and other responsibilities greater than these numerous chores. This we too must do when we reach the maturity appropriate for men. We should abandon earthly things, which in reality are childish playthings [cf. 1 Cor. 13:11], and place before our minds heaven and the splendor of life there and all of its glory.
2. For we have been united with a groom who demands such affection from us that we give up for him not only the things on earth and these small worthless objects but also our own lives when necessary. Therefore, since we must depart for the other world, let us free ourselves from insignificant cares. If we were going to exchange a poor house for a kingdom, we would not care about tiling, timber, utensils and other household needs. So, let us not worry now about earthly things, for the time already summons us to heaven, as St. Paul said in his letter to the Romans: "…For now our salvation is nearer than when we believed. The night is passed, and the day is at hand." [Rom. 13:11-12] And again: "This therefore I say, brethren; the time is short; it remaineth, that they also who have wives, be as if they had none." [1 Cor. 7:29]
3. What good is marriage for those not likely to gain from it, who are in the same plight as those without wives? What good is money? possessions? anything in life, if its usefulness is from now on untimely and inopportune? For if those who are to be brought forward before our court to render an account of their errors think not of their wives, and food and drink, and have no other concern except their defense when the appointed day is near, it is much more incumbent upon us, who are to appear not before a terrestrial but a heavenly tribune to render an account of our words, deeds and thoughts, to give no thought to any­ thing, either present joy or pain, but to concern ourselves exclusively with that fearful day. "If any man," Christ says, "come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea and his own life also, he cannot be my disciple. And whosoever doth not carry his cross and come after me, cannot be my disciple." [Luke 14:26-27]
4. But do you leisurely sit by indulging in passion for a woman and laugh? Are you indolent and given over to luxury? "The Lord is nigh." [Phil. 4:5] Yet do you fret about money? "The kingdom of heaven is at hand." [Mt. 3:2, 4:17, 10:7] But do you look to your house, your life of ease and other pleasure? "…the fashion of this world passeth away." [1 Cor. 7:31] Why then do you wear yourself out in the midst of worldly things that do not last but are used up, and neglect what is sure and lasting? There will no longer be marriage or birth pains, sexual pleasure or intercourse, an abundance of money or the management of possessions, food or clothing, agriculture or seamanship, crafts or construction, cities or homes, but some other system and way of life. All of these will cease to exist in a little while. For this is the meaning of: "…the fashion of this world passeth away." [1 Cor. 7:31] Therefore, why do we exert ourselves in this way as if we will remain here for all ages? Why are we anxious about things that often we will be separated from before evening? Why do we choose a life of hardship when the Christ calls us to one free from strife? "But I would have you," he says, "to be without solicitude. He that is without a wife, is solicitous for the things that belong to the Lord, how he may please God." [1 Cor. 7:32]

St. Paul epistle in 1 Corinthian 7:29-31 is not saying about the end of the world is near rather he is saying that the time of man here on earth is short and temporary. That man should not be lured about the things that are in this world for everything on earth is temporary and passes away.

Please see further explanations of bible commentaries on these bible passages:

Ver. 29.—But this I say, brethren, the time is short. The duration of this life is short, so that we may not think of merely enjoying our wives and the things of this present life, but, as strangers and sojourners, use them for a short time, in order to travel better towards that glorious City into which we shall be enrolled as everlasting citizens. Ambrose takes the time here in a wider sense, as denoting the duration of he world. Time is short, and the day of judgment is at hand: do not, therefore, spend your time on the temporal pleasures of the world, but prepare yourselves for judgment. It remaineth that both they that have wives be as though they had none. That they do not greatly devote themselves to the things of marriage so as to give their spirit, their mind, and their live more to their wives than to the Lord. So Ambrose and Anselm; S. Augustine (de Serm. Dom. in Mont. lib. i. c. xiv.), that they should by mutual consent live in chastity, if possible.

Ver. 30.—And they that buy as though they possessed not. Let them not regard themselves as possessors for ever, but only as tenants for life. Paul is forbidding that inordinate love of things which makes them possess us rather than we them. We are not to fix our heart on transitory things, not with inordinate affection cling to any creature that so soon passeth away.  S. Anselm, S. Augustine (in Joan. Tract. 40), in giving to a rich man a rule for the due use of money, says beautifully: “Use money as a traveller in an inn uses a table, or a cup, or a ewer—as one soon to depart, not to abide for ever.” That God might effectually teach the Jews this lesson, He appointed every fiftieth year to be a year of Jubilee, when all lands that had been sold should return without payment to their first owner. Cf. Lev. xxv. 23. He said to them in effect: “I, the Most High, have true and real dominion over your land; and therefore it belongs to Me to lay down what conditions of sale that I please, especially since I have put you into possession as settlers and colonists, and with you to always remain such. Wherefore I will and decree that all possessions whatsoever return in the year of Jubilee to their first owners, and that for this reason, that you may know, says Philo (de Cherubim), that God alone is the true Lord and possessor of all things, and that men have but usufruct of them, not dominion. “Hence,” says Philo, “it is clear that we use the goods of another; that we possess in the way of right and dominion neither glory, nor riches, nor power, not anything whatever, even if it be some power of the body or faculty of the mind: we merely have the usufruct of them while we live.” Ver. 31.—And they that use this world as not abusing it. By not giving themselves to it overmuch. The Latin version translates the compound word as if it were a simple one—as not using it; but the meaning is the same. Not to use it is to abuse it by holding too tightly to it; for we must use things according to what they are. A world that is fleeting must therefore be used loosely, and by the way as it were, which is as though it were not used. But if you cling to the world you abuse it, for you use a thing that is ever changing, as though it were firm, fixed, and solid. For abuse, as Theophylact says, is use that is immoderate—exceeding the measure and mature of the thing. Hence the Syriac renders this passage, “Let not those that use this world use it beyond its proper measure.” Abuse is found in 1 Cor. ix. 18 in the sense of “use to the full.” Wherefore S. Basil (Reg. Brev. Interrog. 70) says: “The Apostle condemns abuse in the words, ‘use the world as not abusing it.’ The very need that we gave of things that are for use is the measure of their use. He who goes beyond what necessity enjoins is a victim, either to covetousness, or lust, or vain glory.” S. Leo (Serm.5 de Jej. Sept. Mensis) says excellently: “In the love of God is no excess; in the love of the world everything is harmful. And therefore should we hold fast to the things that are eternal, use the things of time in passing, as being pilgrims hastening along the road which takes us back to our country, and regarding whatever good things the world has given us as rather sustenance on the road than inducements to remain. Therefore is it that the Apostle says: ‘The time is short, it remaineth that they that have wives be as though they had them not. &c.; ‘for the fashion of this world is passing away.’ But it is not easy to turn aside from the blandishments of form, of abundance, of novelty, unless in the beauty of visible things we love the Creator and not the creature.” Again (Serm. xi. de Quadrag.), after quoting these words of the Apostle, he adds: “Happy is the man who, in pure self-control, passes the time of his pilgrimage here, and does not rest contentedly in those things amongst which he must walk; who is s guest rather than a master in his earthly home; who does not depend on human affections, not lose sight of the Divine promises.” For the fashion of this world passeth away. The Greek verb may be also translated “is deceitful” or “acts falsely.” For, as S. Augustine says (Ep. xxxix. ad Licentium): “The chains of this world gall while they seem to please, bring certain pain and uncertain pleasure, painful fear and fearful rest; a reality full of misery, and an empty hope of happiness. Will you of your own accord bind your hands and feet with these?” And again (Serm. xxiii. de Verb. Apostol.) he says: “Temporal things never cease to enflame us with expectation of their coming, to corrupt us when they do come, and to torture us when they have gone by. When longed for they enkindle, when obtained they lose their value, when lost they vanish away.” And S. Bernard says “Do not love the things of this world, for they burden us when we have them, defile us when we love them, and torture us when we lose them.” Again, S. Gregory (lib. vi. Ep. ad Andream) says: “Our life is as the journey of a sailor: for the sailor stands, sits, lies down, and is borne along whither the shop carries him. So is it with us: whether waking or sleeping, whether silent or speaking, or walking, or willing or not willing, through the moments of time we are hastening daily to our end. When, then, the day of our end comes, what good will all that do us that we have so eagerly sought after, and so anxiously got together? It is not honour or riches that we should seek after: all these things must be left behind. But if we want to find what is good, let us live those things which we shall have for ever; if we fear what is evil, let us fear those sufferings which the lost suffer eternally.” Then, shortly after, he advises Andrew for the short span of our life and pilgrimage here, “to give himself to sacred reading, to meditate on heavenly words, to kindle himself with love of eternity, to do all good works in his power with his earthly things, and to hope for an everlasting kingdom as a reward for them. So to live is to have a part already in the life of eternity.” S. Jerome says, in his life of S. Hilarion, that “he was wont to remind every one that the fashion of this world is passing away, and that that is the true life which is purchased by the sufferings of this present life.” Fashion. The nature, appearance, and fugitive state of the world, as Ambrose and Anselm say. The Apostle does not attribute form to the world, which is something more firm and constant, but fashion, which is ever changeful, fugitive, and ready to vanish away. Cf. note to Rom. xii. 2. “Do not,” says Anselm, “give the world a constant love; for the object of your love is inconstant. In vain do you firmly fix your heart on it: it flies while you love.” If the world is fugitive, so then is marriage and everything else contained in the world. The day flies by; none knows the morrow’s fount, whether toil or rest it brings: so the world’s glory fades. So too Lipsius, our brother, a man as wise as lifted up above man and human things, was wont with great discernment to say, when we talked together, as we often did freely, of the vanity of knowledge and all human things, that he had long thought of what he would have inscribed on his tomb. It was this: “Do you wish me to speak to you still more loudly? All human things are smoke, shadow, vanity, stage-play, and in one word—nothing.” For all the world’s a play in which this life’s story is given. Men are the platers; they have their exits and their entrances; and the place of the theatre is the earth. “One generation passeth away and another generation cometh, but the earth abideth for ever,” says Ecclesiastes i. 4. On the stage are two doors—that of birth for those coming on, that of death for those going off. Each receives the dress fitted to his part. He who personates a king will not take away with him the purple which he wore. Soon the comedy comes to an end. Seneca says that the same hour which gave us life began to end it. We often hear it said: “Tell me, O farm, O house, O prebend. O money, how many lords thou hast had, and how many yet await thee. Tell me where is Solomon and his wisdom, Samson and his strength, Absalom and his beauty, Cicero and his eloquence, Aristotle and his subtle intellect. Where are the illustrious princes, the things of old, the favour of governors, and strong limbs, the power of the princes of the world?” They are food for worms; they have returned to the dust. Transient as the morning dew, they have fled away. What seek you? What are you so eager for. Happy the man who was able to despise the world! Gregory of Nazianzen enumerates in detail and describes most beautifully and tersely the empty and fugitive nature of everything in this world (de Vitæ Itineribus). He says: “Who am I, and whence came I into this life? and who shall I be, after that having been nursed for a short time in the lap of earth, I return from the dust to life? Where in His universe will God place me? Many are the sorrows that await the traveller on life’s road. and there is no good amongst men unalloyed with evil. And would that evils did not claim for themselves the greater part! Wealth is beset by snares, and the pride of high office and of thrones is the mere dream of a sleeper. To be subject to another’s power is grievous and burdensome. Poverty drags down; beauty is as short lived as the lightning of summer; youth is nothing more than a temporary glow; old age is the gloomy sunset of life. Words take wings, glory is but breath, nobility old blood, strength is shared with the wild-boar, satiety is disgusting, matrimony a bond, a large family is the mother of inevitable anxiety, to be bereaved is as a disease, the market is the seed-plot of vices, rest is feebleness, arts are practised by worthless men, the bread of another is scanty, agriculture is toilsome, the greater number of sailors go to the bottom, one’s native land is a prison, and the region beyond it a scorn.” Then he comprehends them all in one view, and holds up to our gaze the vanity of all things in many apt similitudes, saying: “All things, in short, are full of sorrow for mortals, all human things are fearful and yet ridiculous—like to thistle-down, to a shadow, to dew, to the idle wind, the flight of a bird, to a vapour, a dream, a wave, a ship, a foot-print, a breath; to dust, to a world perpetually changing all things as it revolves—now stable, now rotating, now falling, now fixed by seasons, days, nights, labours, death, sorrows, pleasures, diseases, calamities, prosperity. Not without great wisdom is it, O Christ, that you have so appointed that all the things of this life are uncertain and unstable. Doubtless it was that we might learn to glow with love and desire of something firm and settled, that we might tear away the mind from thoughts of the folly of the flesh, and might preserve pure and intact that image given us from above; might lead a life apart from this life, and, in short, by changing this world for another, bear with fortitude all the difficulties and trials of this life.” S. Augustine too remarks appositely (Enarr. Ps. cx.) on the words, “He shall drink of the brook on the way,” that, “a brook is the current of man’s mortality. As a brook is swollen by the rains, overflows, roars as it goes, hurries along, and as it hurries hastens to its end, so is the whole current of mortality. men are born, they live, they die; and while they die others are born, What stands still here? what is there that does not hasten onwards? what is there that is not as it were collected from the rain, and on its way to the sea, unto the deep?” The fashion of this world implies that it is dressed and masked as an actor. Just as if a man were to sell you a horse and its trappings, you would take off its covering and examine the body and limbs of the horse before buying—even so do here. The world offers you for sale dressed-up honours, masked pleasures, decorated riches. Remove the decorations, take off the mask, look what lurks behind them: you will see that all is foreign, slender, empty. The Wise Man pathetically describes (v. 8) the complaint of the ungodly, and the late remorse that follows on the love of vanity; and he compares it to a slight shadow, a messenger hastening by, a ship cutting the sea, the flight of a bird, an arrow shot forth—to thistle-down, foam, smoke, wind, and to an inn where one spends a night.  S. Jerome explains these images at length in his letter to Cyprianus, in which, commenting on Ps. xc. 4, he says: “Compared to eternity the length of all tome is short.” Then, at ver. 6, he says: “As in the morning the grass flourishes, and delights with its verdure the eyes of all that see it, and then gradually withers and loses its beauty, and is turned into hay to be trodden under foot, even so does the whole race of men show the freshness of spring in childhood, blossom in youth, and flourish in manhood; but suddenly, when he knows not, the head turns white, the face wrinkles, the skin contracts, and at last, in the evening of old age, he can scarcely move. He is hardly recognised for what he used to be, and seems almost changed into another man; and, lastly, as Symmachus turns Ps. xc.10, we are suddenly cut down and fly away.”

Ver. 32.—But I would have you without carefulness, and therefore living in virginity and celibacy.