sub join_channel { ... }
  sub part_channel { ... }
  sub msg_user { ... }

  my %dispatch = (
    join => \&join_channel,
    part => \&part_channel,
    msg  => \&msg_user,
  );

  sub bot_command {
    my $cmd = shift;
    $dispatch{$cmd}
       ? &{$dispatch{$cmd}}
       : &default
  }