Medeans [may go out on Shabas] with p'rifa. Anyone [may], but the rabbis spoke about what occurs.
That is, Medeans do p'rifa, so the rabbis mentioned that they can go out like that on Shabas. Rashi explains the p'rifa was a sort of button: one would wrap a hard object, like a stone, in one edge of a garment and a strap to the other edge, put the garment around herself, and wrap the strap around the hard object to keep the garment in place.
The mishna continues:
One may do p'rifa with a stone, a nut, or a coin, so long as she does not do p'rifa first on Shabas.
The g'mara there (amud 2) notes the obvious problem with "so long as she does not do p'rifa first on Shabas":
But you said in the earlier clause that "one may do p'rifa"!?!
Abaye (there) resolves the difficulty:
In the later clause, we come to a coin.
That is — Rashi explains — the prohibition on p'rifa applies to a coin, which is muktze and cannot be handled on Shabas, whereas the permission immediately preceding it applied to things that are not muktze.
Huh?? The entire relevant difference, it seems, between the second-to-last and last clauses — the entire reason that the former permits and the latter forbids — is that "in the later clause we come to a coin". But the second-to-last, permitting clause explicitly mentions a coin!