The Piper is a metaphorical figure, like the Grim Reaper, who represents karma, accountability, a final reckoning for one's misdeeds, in short He Who Must Be Paid. He appears on your doorstep in motley, wearing bells and a foolscap. You can run from him but you can't hide--sooner or later he will find you and make you pay for your antics.
"Joe thought he could bang that ugly girl and then slip away in the morning and never have to deal with her again. Now she's calling him ten times a day. Joe's paying the Piper."
A mythological or fictional being that collects dues while playing the flute after an evening of heavy alcohol or drug use. Bringer of the hangover.
(1) I woke up with a very bad headache, I felt nauseous, and I think I saw the piper in my kitchen. (2) On the way home from 111 Minna, I could hear the faint sound of his zampona and I knew I'd have to pay the piper.