> Perl 6 has an asynchronous looping construct called whenever ... runs whenever a value arrives ... can live in a ... react block (works like entering an event loop)

  my $code = supply {
    whenever IO::Notification.watch-path($src-dir) {
      emit .path if .path ~~ /<.pm .p6> $/;
    }
  }

  react {
    whenever $code -> $path {
      say "Code file $path changed!";
    }
  }

The "react" keyword is just a channel subscription, and inside it has a pattern matcher. Channels are first-class citizens, and you're encouraged to chain channels rather than falling into callback hell. It's like Go and Ruby had a child.

If anything piqued my interest in Perl 6, it was this.