http://help.sap.com/abapdocu_702/en/?url=ABAPOPEN_DATASET.htm
oh, you forgot LOWER CASE in the PARAMETERS. The consequence is that the full access path is changed to upper case i.e. /USR/SAP/TRANS/ECC/TMP/POONAM.TXT, which surely is not valid (USR doesn't exist, but usr exists.