$muc->reg_cb(
  message => sub {
    my ( undef, undef, $msg ) = @_;

    my @connected_users = ();
    open my $fh, '-|', 'command_that_outputs_users' or die $!;
    my $w; $w = AnyEvent->io(
      fh   => $fh,
      poll => 'r',
      cb   => sub {
        my $user = $fh->getline;
        return if !defined($user);
        chomp($user);
        push(@connected_users, $user);

        if(eof($fh)){
          close($fh);
          undef $w;

          open($fh, "< $file") or die $!;
          my $w2; $w2 = AnyEvent->io(
            fh   => $fh,
            poll => 'r',
            cb   => sub {
              my $user = $fh->getline;
              return if !defined($user);
              chomp($user);

              if($user ~~ @connected_users){
                AnyEvent::XMPP::IM::Message->new(
                  body => construct_body( $msg->body, $from ),
                  to   => "$user\@" . HOST,
                  type => "chat",
                )->send($conn);
                close($fh);
                undef $w2;
              }
            }
          );
        }
      }
    );
  },
);