Drilling minor holes on the wall may be considered as structural damage which goes beyond the scope of normal wear and tear. This may result in deductions by your landlord from your security deposit upon vacation unless a written permission is taken from him. The landlord may claim deduction for maintenance and repair as the tenant has the obligation to return the apartment in the same condition as was given at the time of entering into the contract, except for ordinary wear and tear or damage due to reasons beyond tenant’s control. Read more here