Dear Anjali
It depends upon the rules and policies of your organization. If there is a written policy on absenteeism, then you can imply the rule. But if not or if the employee didn't know that he/she had to inform the management, then as a HR manager and as the custodian of the intangible assets i.e. the human capital or the employees, you can make them understand.
So, as Mr. Mahesh Sharma explained in his post, you can do that.
When the employee comes back from the leave, you may have a one to one chat with him. Explain politely but firmly that every employee need to inform the management about his/her leave. Also, you may tell the employee the consequence of not taking prior permission.
Simultaneously, after the one to one discussion, write a mail to the employee giving all the details of the face to face meeting. This way, the management will also be in the loop (if they ask you what have you done).
Regards