"That she would one day abandon country has long been clear. It’s a big box, and a porous one, but a box all the same. “1989” (Big Machine), though, her fifth album and the first that doesn’t at all bother with country, manages to find a new foe. Full of expertly constructed, slightly neutered songs about heartbreak, “1989,” which is to be released on Monday, doesn’t announce itself as oppositional. But there is an implicit enemy on this breezily effective album: the rest of mainstream pop, which “1989” has almost nothing in common with. Modern pop stars — white pop stars, that is — mainly get there by emulating black music. Think of Miley Cyrus, Justin Timberlake, Justin Bieber. In the current ecosystem, Katy Perry is probably the pop star least reliant on hip-hop and R&B to make her sound, but her biggest recent hit featured the rapper Juicy J; she’s not immune.