The zabbix_sender program needs command line options for this script to work.
I would also update the ZABBIX_SERVER and ZABBIX_PORT to be more sane.

The original script:
ZABBIX_SERVER="zabbix";
ZABBIX_PORT="10001";
$ZABBIX_SENDER $ZABBIX_SERVER $ZABBIX_PORT $HOST $KEY "$str"

The modified script:
ZABBIX_SERVER="localhost";
ZABBIX_PORT="10051";
$ZABBIX_SENDER -z $ZABBIX_SERVER -p $ZABBIX_PORT -s $HOST -k $KEY -o "$str"