Fiscal Year to Date filter
Below code fully meets the requirement:

PER_PERIOD_START_DATE between 

(CASE
WHEN (PER_MONTH_NUM = 10 and extract( year from PER_PERIOD_START_DATE) = 2018)
THEN Add_Months(Trunc(Add_Months(sysdate,3),'YYYY'),-3) + 14
ELSE Add_Months(Trunc(Add_Months(sysdate,3),'YYYY'),-3) 
END
)
and trunc(sysdate, 'MM')-1


Alternative:

PER_PERIOD_START_DATE between 

(CASE
WHEN (PER_MONTH_NUM = 10 and extract( year from PER_PERIOD_START_DATE) = 2018)
THEN '15-OCT-'||(EXTRACT (YEAR FROM ADD_MONTHS(sysdate, -9)))
ELSE '01-OCT-'||(EXTRACT (YEAR FROM ADD_MONTHS(sysdate, -9)))
END
)
and trunc(sysdate, 'MM')-1