From the information you provided
These are the main reasons for perianal itching
1. Worms cause itching but as you have already taken medication for that I assume persistent problem is not due to worms. ( Unless medication you took is sub-optimal) you can get it confirmed by a stool test in case of doubt.
2. Fungal infection due to sweat/humidity or compromised immunity.
3. More information will be required to completely understand problem and treat it completely
Next Steps
keep area dry and apply easily available sweat absorbing powder.
maintain good hygiene
avoid sitting on hot seat of bike/car or long sitting on leather/plastic chair - use cloth cushion.
Health Tips
follow above and keep a watch on timing of itch.
morning/ night or during strenuous workout etc.
if problem does not get resolved soon, must consult a doctor clinically to rule out worm/fungal infection /dermatitis or perianal abcess.
all the best.