DACCA, PAKISTAN. - Going to East Pakistan after a week in West Bengal is like walking through the looking glass.

Suddenly everything is reversed. What you were told in Calcutta about fleeing refugees becomes untrue in the Dacca version of events.

Similar reversals apply to all the other great problems that beset Pakistan today. You begin to wonder which side is the world of reality and which of fantasy - and whether the Mad Hatter is the information officer of the Pakistani or Indian side of the border .

This might be ludicrous if so much were not at stake on both sides. There is the plight of the miserable tide of humanity which swept out of East Pakistan into the eastern provinces of India.

And there is Pakistan's reputation before the world, which has been heavily impugned during the recent months of internal strife and dislocation.

So one comes here - among the first foreign correspondents to be readmitted - to see for oneself.

WHAT REFUGEES?

Here and in Karachi what everyone in India calls refugees are not admitted to be refugees at all. They are called displaced persons - "poor, mistaken people who listened to false propaganda, then panicked and fled out of sheer fear when they need not have left at all."

Here they say displaced persons are welcome to return regardless of caste, creed, or religion Pakistan's President Yahya Khan himself has said so. And some are indeed coming back - according to government reports - although the numbers still are in the low thousands compared with the millions who fled.

India, however, is accused here of holding refugees by shipping them to the interior on the grounds that border area camps are overcrowded.

To Dacca from Karachi by air now takes over six hours - almost as long as a North Atlantic crossing - because Pakistan's planes must detour around the entire Indian subcontinent to span the 1,200 miles between the two wings of Pakistan.

Below, the fields shimmer with water, and one remembers the great cyclone and tidal wave that devastated the Ganges estuary only last November.

At that time the storm seemed the greatest calamity that could hit this area. Yet within months this catastrophe of nature was exceeded in misfortune by a manmade disaster that already has altered the lives of millions, brought India and Pakistan close to conflict, and threatened the stability of South Asia.

At the Dacca air terminals where last November throngs of Pakistani spectators crowded airport corridors and lined the railings of it rooftop lookout both night and day. there now are no spectators.

Dacca Airport is closely guarded and restricted to persons actually arriving and departing: It no longer is for those who wish to watch in idle curiosity planes and passengers coming and going.

On the way into town taxi fare has doubled in the past six months. The driver complains that gasoline prices have risen and business is poor. Traffic is half what it was formerly and all the roadside clutter and jumble of carts, cars, buses and stalls has been swept away. It is neater, quieter, cleaner - but it is not the Dacca of old.

DACCA NOT "DESERTED"

Dacca, they said in Calcutta, is a deserted city. This is not true. It is not deserted: but it does appear greatly diminished in population and activity. Downtown traffic is light compared with before the trouble started.

Such surface differences as these are easy to spot and perhaps not all that significant.

Much more difficult is sorting out the serious but utterly divergent claims of Pakistan's government on one side and its critics in Calcutta and New Delhi on the other.

Pakistanis feel they have had a bad press in the outside world. They realize some of this is due to the high- handed manner in which the foreign press was ejected from Dacca at the height of the Bangla Desh uprising. A few may even concede that the restrictions of prolonged martial rule had something to do with it.

But now the government has decided to let overseas newsmen in again to see for themselves. There has, of course, been considerable time for the Army and military government to tidy things up and prepare cogent answers to obvious questions.

"I hope you write the truth about what you see in East Pakistan," said the girl who wrote my airline ticket in Karachi. "You Americans have been biased against us so far."

She had reached her judgment on the basis of controlled Pakistan press reports and reading two United States news magazines every week .

"I hope you won't write about Bangla Desh in your stories."

A British businessman just returned from Dacca said in Calcutta, "There is no Bangla Desh except in the imagination of some political exiles in Calcutta. East Pakistan is a going concern today. Bangla Desh is not." It is true Bangla Desh is not visible as a going concern on East Pakistan soil at present. But it manages to make its presence known nevertheless.

BOMB BLAST DESCRIBED

Only moments before, the same businessman had been telling me how he and his chum narrowly missed injury Just outside their Dacca hotel when several grenade bombs went off. They had bits of shrapnel to show for their near miss.

Somebody out there in the night was making known his Bangla Desh resistance sentiments even at this late date. The hotel now has security police on constant duty.

As further evidence of some continuing Bangla Desh activity, one also hears - without confirmation - that a small but steady stream of military and civilian casualties reach Dacca Hospital daily.