What should I do about 0.5 inch septal perforation?

Thanks for the question.
It is unfortunate that you have a perforation following surgery. 
Only symptomatic repairs need closure. I would suggest Neil Med sinus rinse 2-3 times a day and sniffing up Vaseline to lubricate the nose. If, this settle your symptoms then I would suggest you could avoid surgery. If however you have crusting, bleeding, sensation of blockage, noisy breathing it would be worth considering closure. A 0.5 cm perforation is amenable to closure with high success rates. I would delay the repair for at least 1 year following. If the perforation is asymptomatic I would get regular review (yearly) to check the perforation is not enlarging which can occur in some perforations. If, it was enlarging I would recommend closure before the perforation increases so much to make closure difficult. I hope this helps.
Dr Callum Faris

Septal perforations are a known complication of septal surgery, and in some very deviated septum’s are difficult to avoid.  Only if it results in symptoms, such as crusting, bleeding, saddle nose deformity, or persistent whistling, do they need to be repaired.

Septal perforation is a known complication of septoplasty, sometimes with the best of surgeons.  Perforations are more common in smokers, nose pickers and cocaine users. If a perforation is without symptoms, no treatment is necessary. If there is crusting, whistling or obstruction, repair is indicated. You will probably need to wait a year from your last surgery before repair.  See a rhinoplasty expert with experience in septal perforation repair. Good luck!