ok. the problem is in the sql statement (I don't know why!). If you
use a store procedure:

CREATE PROCEDURE TEST
RETURNS (
V1 Char(100),
V2 Char(100))
AS
DECLARE VARIABLE temp CHAR(100);
BEGIN


FOR SELECT A_FILD_CHAR FROM A_TABLE INTO :v1
DO BEGIN
temp = UDF_STR2STR(v1, 20);
v2 = UDF_STR2STR(temp, 20);
SUSPEND;

END


END;

you will see that v1 = v2.