sub DBthread {
   my( $Qin, $Qout ) = @_;
   my $dbh = DBI->create( ... );
   while( my( $tag, $request ) = split ' : ', $Qin->dequeue ) {
        my $sth = $dbh->prepare( $request );
        my $response = $sth->execute;
        $Qout->enqueue( "$tag : $response );
   }
}

my( $QtoDB, $QfromDB ) = map{ new Thread->Queue } 1 .. 2;
my $threadDB = threads->create( \&DBthread, $QtoDB, $QfromDB );
...
my $listener = new IO::Socket( .. );
my $sel = new IO::Select;
my $sel->add( $listener );

my %clients;

while( 1 ) {
    ...
    for my $fh ( $sel->can_read ) {
        if( $fh == $listener ) {
            my $client = $listener->accept;
            $sel->add( $client );
        }
        else {
            $fh->recv( $buf, 100 );
            ...
            $QtoDB->enqueue( "$fh : select something from somewhere" );
            $clients{ $fh } = $fh;
        }

        while( $QfromDB->pending ) {
            my( $tag, $response ) = split ' : ', $QfromDB->dequeue;
            my $clientFH = $clients{ $tag };
            $clientFH->send( $response );
            $clientFH->shutdown( 2 );
            $sel->delete( $clientFH );
        }
    }
}