Will Perl6 be BNF-able?
Not on your life. In Perl6 you can rewrite the
very grammar of the language ("All's fair if you
predeclare"), and there's absolutely no way BNF
can handle that.
"In adjectives, with the addition of inflectional endings, a changeable long vowel (Qamets or Tsere) in an open, propretonic syllable will reduce to Vocal Shewa. This type of change occurs when the open, pretonic syllable of the masculine singular adjective becomes propretonic with the addition of inflectional endings."
— Pratico & Van Pelt, BBHG, p68