use Plack::Request;
   use DBI;   
   use Conf;
   use Modern::Perl;

   my $env = shift;
   my $qd  = $req->parameters->mixed;

   my $dbh = DBI->connect(
  
          $Conf::conf->{'dsn'},
          $Conf::conf->{'dsn_username'},
          $Conf::conf->{'dsn_password'}
      
      ) || die "Could not connect to 
                  database: $DBI::errstr";

  my $sth = $dbh->prepare(qq@ 

        SELECT count(1) FROM users 
        WHERE username="$qd->{'username'}"

      @);

  $sth->execute;

  ....
  ....