my $timenow = time();
  my $fiveminutes = 5 * 60; # 60 seconds times five
  my $timethen = $timenow - $fiveminutes;

  my $sql = "DELETE FROM mytable WHERE lasttime < ?";
  my $sth = $dbh->prepare( $sql );
  if ( $sth && $sth->execute( $timethen ) )
  {
      ; # success
  }
  else
  {
      die( "DB Error " . $dbh->errstr . "\n" );
  }