Should not custom be given consideration in validating a perpetual lease contract, such that a specific duration, such as 100 years, is determined instead of annulling the contract, especially since the property owner's intention at the time of drawing up the contract was perpetuity?

If the duration is not specified in a lease contract, the contract is invalid, and it must be فسخ (annulled). Then, the two parties should enter into a new contract in which they specify the duration. It is not permissible to oblige the lessor to a duration that he does not agree to. As for the period that has passed under the invalid contract, a fair market rent (أجر المثل) must be paid for it.