Perez de Cuellar's turn?

Has the time come to enlist United Nations Secretary General Javier Perez de Cuellar in a last-ditch diplomatic effort to avert war over the Falklands?

To be sure, the UN has not always been effective in resolving cases of unilateral seizure of disputed territory. But, with the energetic Haig mission showing no results and the crisis nearing the point of armed conflict, the Security Council might play a useful role. Nothing could be lost - and perhaps much gained - by empowering Mr. Perez de Cuellar to step in and see what he can do.

While one sympathizes with the need to reassert British rule in the Falklands and to take account of the islanders' wishes, there are strong arguments for avoiding a major confrontation. Not only could there be heavy loss of life. But such a clash would make continuing diplomatic efforts even more difficult.

In light of the secretary general's Latin background (he is Peruvian), a mediating role would not have been possible before. The British might not have regarded him as an unbiased go-between. But the situation has now markedly changed. The Organization of American States itself has urged an immediate truce in the British-Argentine dispute. It has also stated that the terms of UN Security Council Resolution 502 on the Falklands have to be carried out ''in all its aspects.'' These terms call for Argentine withdrawal from the islands, cessation of hostilities, and a diplomatic settlement.

Argentina has thus been rebuffed by the very nations it thought would come to its aid. It did receive moral support: the OAS backed Argentina's historical claim to sovereignty over the islands and criticized the economic sanctions imposed on Argentina by the Common Market countries.But there was reluctance to take tough action. Colombia went so far as to call the Argentine takeover of the Falklands a ''military occupation'' and to describe the British response as ''not entirely an act of outside aggression.'' With many other territorial disputes simmering in Latin America, many OAS members seemed to be sensitive to the pitfalls of supporting armed action to resolve them.

This should give the government of General Galtieri pause. While Argentina ostensibly stands to lose face if it backs down, a statesmanlike position would recognize the greater folly of pursuing a reckless course. To reverse such a course is no cause for shame but for honor. Argentina's position at the moment is unenviable. It has lost stature in the international community and lacks support even among its immediate neighbors. It is forcing the United States to tilt solidly behind Britain if full-scale hostilities break out. Many of its own citizens have misgivings about the invasion and question whether the islands are worth shedding blood for. Then there is the high toll of war if it comes - not only in lives and economic losses but in deepened national animosities and a fraying of relations with Europe.

From the standpoint of the Galtieri government, it might be politically difficult to agree to withdraw the Argentinian troops, avoid a clash, and then begin negotiations. But Argentina is likely to gain sovereignty over the islands sooner by this patient diplomatic route than by taking on the British task force, a move which, ironically, would only prolong the conflict and defer settlement of the issue by legal means.

Would Britain agree to a UN mediating effort? Margaret Thatcher has rejected an appeal by opposition Labour leader Michael Foot to go back to the United Nations, so this poses a political problem for her. But, inasmuch as Britain has the UN on its side, Mrs. Thatcher should be willing to take the political heat of acquiescing to a new diplomatic effort. There can be no pride in war if war can be avoided. Mr. Perez de Cuellar should be given a chance.