The Shulhan Aruch in Siman 312 dedicates ten Halachot to a situation in which one uses the bathroom out in a field on Shabbat and needs to clean himself. While this is not common today, it was very relevant in those days in which people did not have private bathrooms. In those days, it was the practice to use certain rocks which were suitable to wipe themselves clean. These Halachot deal with the problem that a rock is Mukse. Ultimately the Hachamim waived the issue of Mukse in the face of the need for hygiene because of the importance of Kavod HaBriyot-human dignity.
The practical application of this issue today, although not related to Mukse, is what to do if one does not have cut toilet paper in the bathroom. All the Poskim are clear that one must ensure that he has pre-cut paper before Shabbat. The question arises what to do in a case where one does not have pre-cut paper on Shabbat. Hacham Bension (Or LeSion II, 40:6) does not allow tearing the paper, even with a Shinui-in an unusual fashion, which is only prohibited M'Drabanan (rabbinically). Although the Hachamim waived the Rabbinic prohibition of Mukse in the face of Kavod HaBriyot, tearing with a Shinui is more severe, since it is rooted in the Torah prohibition of "Mehatech"-the Melacha of cutting.
Hacham Ovadia (Yabia Omer OV 9:108:185) argues with Hacham Bension and says that this is not a case of "Mehatech", because that is only when a person is interested in cutting the paper to a certain size to achieve a certain finish. In this case, one does not care about the size of the paper; the perforations are only there to make it easier to cut. Therefore, if person finds himself in a situation without pre-cut paper, he may tear the paper normally, even without a Shinui. The Yalkut Yosef and the Menuhat Ahaba, do recommend that one cut the paper with a Shinui, such as with his mouth or foot.
SUMMARY
One should prepare pre-cut tissues before Shabbat. In the even that one did not do so, it is permitted to cut the paper with a Shinui.
