The Pentagon's top arms control specialist said yesterday that the administration's continuing toughness in talks with Moscow was one reason the Soviets have proposed missile cuts below the levels of previous arms limitation agreements.

"It's time for some people to reconsider some hasty judgments about how effective the president's approach is in dealing with the Soviet Union," Assistant Secretary of Defense Richard Perle yesterday told reporters at a breakfast meeting.

"The Soviets are talking to us about levels lower than SALT II," he said, referring to the U.S.-Soviet strategic arms limitation agreement signed in 1979 but never ratified by the Senate. The new levels, he said, "are not a lot lower, but they are lower."

That, he added, "is movement in the right direction," even though "we've got a long way to go" before the talks get to the question that most concerns the United States, which is the striking power rather than just the size of the Soviet missile force.

Perle, who is in charge of international security policy at the Pentagon, said whatever movement there has been in the Soviet position "has not been significant" because it has not dealt with the Soviet's large SS18 and SS19 missiles--some 640 in all--that U.S. planners say threaten the American land-based missile force and that the United States wants sharply cut back.

Perle's remarks were made as he was questioned about a report that the Soviets last week modified their position at the strategic arms reduction talks (START) in Geneva. That Washington Post report said Moscow has now proposed reductions in long-range, multiple-warhead, intercontinental ballistics missiles and bombers below the levels of SALT II. But the new proposal reportedly would let the Soviets keep all their big missiles while reducing other forces.

White House and State Department spokesmen yesterday acknowledged that "during this round of START the Soviets have provided additional information elaborating on their proposal. We hope that the further information . . . indicates a Soviet intention to show similar flexibility to ours and to move the negotiations forward."

Perle's comments yesterday on several aspects of the U.S.-Soviet battle over nuclear arms control reflect the unusual stage that this struggle appears to have reached.

For example, the view is widespread in the administration that the Soviet shifts at Geneva thus far, as Perle said, have not been significant in that they have not dealt specifically with the most threatening Soviet forces. Yet officials want to take credit for whatever progress has been made, which they attribute to President Reagan's toughness in seeking big cutbacks in arms while proceeding with a defense buildup.

Several officials said the administration is "more receptive than ever," as one put it, to any serious Soviet offer to limit numbers of warheads and the largest missiles, as the U.S. has proposed. Still, there is no sign yet that such a move will occur because the U.S. proposal calls for vastly greater shifts in Soviet rather than in U.S. forces, in part because the Soviet missile forces are larger.

"The Soviets have some straightforward calculations to make," Perle said, noting that the United States has an array of new weapons under development, including the MX and Trident II missiles, two new bombers and new cruise missiles.

"If I were the Soviets, I'd be inclined to conclude that this is the right time to agree on the kinds of ceilings we are proposing because they'll be better off and the world will be more stable."

Another paradox is that while Washington is publicly stressing its flexibility at the arms talks it also has under way a study of alleged Soviet violations of previous arms agreements.

While Reagan has been reluctant to accuse the Soviets publicly of specific violations, there is widespread agreement in the administration, as Perle also hinted, that new Soviet missile tests and the encoding of data transmitted to the ground during those tests are violations of the spirit and possibly the letter of previous pacts.