Subtlety is key when sipping tea at another celebrity, especially one with whom you've had previous public beef. And Katy Perry is here to teach you how it's done.In case you've forgotten, Perry and fellow pop star Taylor Swift are not on good terms.The two went from good friends (they hung out at the 2010 Grammys) to awkward acquaintances (they both dated John Mayer ) to absolute enemies (Perry apparently stole Swift's backup dancers ). Their feud has pretty much been on the DL as of late, but Perry is here to give you a quick update on their hot-and-cold relationship : It's still very much on the frigid side.The 31-year-old singer debuted her latest fragrance on June 21, calling it "Mad Love."Sound familiar? It's also a memorable phrase from Swift's 2015 hit "Bad Blood."Upon its release, the song was rumoured to be all about the squashed friendship between Swift and Perry. On the track Swift sings, "Cause baby, now we've got bad blood / You know it used to be mad love / So take a look what you've done / Cause baby, now we've got bad blood, hey!"Perry is throwing some major shade by taking Swift's own lyric and slapping it on her perfume bottle. Granted, the first fragrance in Perry's collection is called "Mad Potion," but we all see the double meaning of her latest launch. It's pretty shady.Sure smells like trouble to me.