A lousy interface on your module irritates users, and could scare off some entirely. It doesn't matter how lovely your class, method, or function is internally — how users invoke it and get data back from it is what will affect them most, whether you're writing a module for Cpan or internal library routines for colleagues.
Smylers presents a collection of avoidable interface infelicities encountered in the wild, in the hope that we can all agree to never ever repeat them ever again in software we write. Ever.