PL/pgSQL examples NOT involving functions

How can I write a few lines of PL/pgSQL which do not involve creating
a function? I can find no examples of this in the docs, but say I
would like to do something like

BEGIN
IF EXISTS (SELECT * FROM foo WHERE idx = 27)
THEN
UPDATE foo SET var='some value' WHERE idx=27;
ELSE
INSERT INTO foo (idx, var) VALUES (27, 'some value');
END IF
END;

roland
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