Basing MX land missiles in tight formation in the so-called "dense pack" might impel the Soviet Union to deploy blockbuster warheads of 20 megatons to wipe out all of them simultaneously, the Pentagon's former research director said yesterday.

William J. Perry, military research director during the Carter administration, said the dense pack also might be vulnerable to what is known as the "pin down" effect. This results from explosion of one Soviet missile after another over the small dense-pack field so MXs could not be fired without being destroyed by the lethal radiation umbrella.

Perry, who favors spreading MX missiles far apart to make them difficult to destroy, said he has not rejected the dense-pack proposal but is seeking Air Force answers to questions he raised during a breakfast meeting with reporters yesterday.

He added that he was "very much" in favor of sticking with limits established in the U.S.-Soviet arms control agreement known as SALT II, which has never been ratified.

Dense pack, as described by the Air Force, would require digging new holes for the MX that would seem to violate SALT II prohibitions. To avoid a violation, Perry said, the United States must negotiate a deployment scheme acceptable to the Soviets.

Administration officials confirmed yesterday that President Reagan intends to abide by SALT II, if the Soviets do, but they did not offer a time limit. Since MX will not be ready for deployment before 1986, such a pledge would not inhibit manufacture of the missile or refinement of the dense-pack scheme in the meantime.

Perry said the Pentagon told president Carter that an alternative to deploying MX was to settle for putting more nuclear firepower at sea in submarines and building more cruise missiles for aircraft.

The current force of Minuteman land missiles would have been left intact under that option, he said, with the possibility that they could be replaced eventually with a version of the Trident II submarine missile.

In discussing the Falkland Islands conflict, Perry said the British need most a combination of U.S. Airborne Warning and Control System radar surveillance planes and long-range fighters such as the U.S. Navy's F14 Tomcat.

Perry acknowledged that the British have the Nimrod, their version of the AWACS plane, but said it lacks the sophistication of the U.S. craft.

An AWACS plane could be used to spot Argentine aircraft more than 100 miles away and guide the RAF to them, he said. What the Argentines need most, he continued, are modern anti-ship submarines such as the U.S. nuclear-powered Los Angeles class.

Rather than Argentine aircraft, "a few first-class subs would be the ones sinking the British ships," Perry said. Although the Argentines have a few subs, they do not compare with those in the Los Angeles class. Argentine fighter-bombers armed with old-fashioned bombs and smart Exocet missiles have inflicted most of the damage on the British fleet.