The visarga is the substitute when it is followed by a hard consonant (khar) which itself is followed by a sibilant (śar).,
The word शर्परे is a Bahuvrihi, meaning that which is followed by शर् ॥ In other words, when a sibilant follows a hard consonant, the preceding visarga remains unchanged. Thus शशाः क्षुरम्, पुरुषः क्षुरम्, अद्भिः प्साताम्, वासः क्षौमम्, पुरुषः त्सरुः, घनाघनः क्षोभणश्चर्षणीनाम् ॥
Though the sutra could have been shortened by saying शर्परे न; yet the longer form is used, in order to indicate that the jihvamuliya and upadhmaniya changes also do not take place, in cases like अद्भिः प्साताम्, वासः क्षौमम् ॥,