Instead of sweltering in the central jail, where Mushtag Malik was supposed to be serving a three-year sentence at hard labor as one of Pakistan's leading drug smugglers, he was to be found lolling in an air-conditioned hospital room.

Malik reportedly offered visiting journalists coffee and cold drinks, called his police guard "my servant," and bragged of being able to ship pounds of heroin around the world from his hospital room because "I can buy anyone in this country."

"You can see with your own eyes, I still live like a prince," Malik, popularly known in this seaport city as "the black prince," was quoted as saying.

As a result of the widespread publicity in Pakistani papers about his high living while sentenced to hard labor, Malik was returned to jail. The government said he had "misused the facility of hospitalization."

But the ease with which he escaped punishment and the audacity of his claims to special treatment showed how sluggish is the war against drug trafficking in this country.

Pakistan is the center of the "Golden Crescent" region of Southwest Asia that U.S. drug enforcement officials estimate supplies at least half of the heroin sold in the United States. As much as 80 percent of the heroin sold in Western Europe also comes from the region, said enforcement officials here and in Pakistan's capital city of Islamabad.

Last Monday, a U.S. district court in Washington sentenced a Pakistani, Javed Nawaz, to 18 years in prison in connection with a plot to smuggle $20 million worth of heroin from Pakistan to the United States.

U.S. drug enforcement officials estimate that Pakistani farmers produced 54 tons of opium last year, which could be converted to 4.8 tons of super strong heroin. By the time it is cut to reduce its potency, it will amount to 10 to 12 times that much heroin for sale on American streets.

Even more heroin is produced in the rugged mountains of Afghanistan, adjoining Pakistan, and transported through this country to the West.

With a war raging in Afghanistan, estimates of its opium production are uncertain. They vary from 144 tons to 480 tons last year.

Although there is no proof, many western diplomats suspect that Afghan guerrillas fighting Soviet-backed government forces grow opium poppies in the border areas they control. Poppies are a perfect crop for an area wracked by war, since they need little tending.

Iran is the third country in the Golden Crescent, which in the past six years has challenged the "Golden Triangle" of Southeast Asia as the major source of heroin. Iran is believed to have cut its production of opium drastically as a result of severe penalties instituted by the Islamic fundamentalist rule of Ayatollah Ruhollah Khomeini.

Pakistani President Mohammed Zia ul-Haq's martial-law government, pressed by the Reagan administration and some members of Congress -- who threaten to cut aid to Pakistan because of its inaction on drugs -- appears to be trying to stop the flow of heroin to the West.

The efforts are hampered by widespread corruption in Pakistan's law enforcement apparatus that has intensified as a result of the large profits that can be made from heroin sales, U.S. officials said.

Malik, in his hospital interview with two West German television correspondents and a Pakistani reporter, blamed western narcotics agents for corrupting the nation by paying large sums to informers.

"The traffickers have so much money now that they can buy a lot of influence here," said a senior U.S. official, one of two dozen in Pakistan who deal with the narcotics problem.

"Big and influential people are behind the massive heroin trafficking," said Sen. Amir Abdullah Khan Rokart during a floor debate last June in which he charged that there was police corruption.

Two major drug traffickers, arrested at about the same time as Malik, mysteriously got out of jail after being convicted and sentenced to three years at hard labor for trying to smuggle heroin abroad.

The two, Feroz Gotto and Akbar Ali Dosa, were spotted two months later on the streets of Karachi by informants for U.S. drug agents. The two were sent back to prison only after U.S. Ambassador Deane R. Hinton and Jon R. Thomas, the assistant secretary of state for narcotics who was visiting Pakistan in June, brought the information to President Zia's attention.

Zia was reported to have been furious when he heard the two men were out of jail. "How is that possible?" Zia was said to have asked his interior minister.

Earlier this year, U.S. officials said, Zia intervened at Hinton's request to force a speedy and honest trial of a deputy superintendent of police in Lahore who was charged with using his influence with airport security forces to allow carriers of heroin to avoid customs searches.

The police superintendent, Khalid Tikka, headed an anticorruption unit when he was arrested on the drug charge in February. According to U.S. sources, Hinton brought that case to Zia's attention twice before Tikka was convicted and sentenced by a martial-law court to three years at hard labor.

U.S. undercover agents have told of instigating drug purchases only to find that Pakistani police who were to come in and make arrests at a prearranged signal failed to do their job. The U.S. officials say they are unsure whether the failure was due to corruption or incompetence.

According to these sources, the Pakistani government dragged its feet on a U.S.-funded program to take aerial photos of the opium-growing regions. Hinton reportedly had to go to Zia at least twice to clear away military and bureaucratic objections to the survey, part of an effort to eradicate poppies and shift farmers to other crops.

Despite these problems, Pakistan has won some rounds in its contest against drugs. The political agent in the Khyber Pass, Javed Alam Khanzada, ordered a raid last November by police and a detachment of the famed Khyber Rifles against what Drug Enforcement Agency officials called the largest opium-to-heroin conversion laboratory in the country.

In March, Khanzada led the Khyber Rifles in an attack on the home of a leading Afridi tribesman of the frontier area, Walid Khan Kukkikhel, who had been trying to persuade farmers to grow opium poppies. In the battle with the tribesmen, the troops blew away Kukkikhel's house, believed to contain two heroin processing labs, U.S. officials said.

The most potent factor in forcing Pakistan to fight the drug problem may turn out to be the sharp increase in the number of heroin users among the country's middle- and upper-class youth.