execute block 
as 
begin 
    execute statement 'SELECT * FROM MYTAB'
    on external 'C:\123\ATEST.FDB'
    as user 'SYSDBA' password 'masterkey'
    with autonomous transaction;
end

Unsuccessful execution caused by system error that does not preclude successful execution of subsequent statements.
Output parameters mismatch.

EXECUTE BLOCK
as
DECLARE VARIABLE Q_FIELD_NAME VARCHAR(100);
begin
  FOR
    EXECUTE STATEMENT 'SELECT FIL FROM MYTAB' 
      ON EXTERNAL 'C:\123\ATEST.FDB'          
      WITH autonomous transaction
      AS USER 'SYSDBA' PASSWORD 'masterkey'   
      WITH CALLER PRIVILEGES
    INTO :Q_FIELD_NAME
  DO INSERT INTO TABL (MFSTR) VALUES (:Q_FIELD_NAME);  
end