$q->param('siteid') =~ /^\s*(\d+)\s*$/
my $site_id = $1

if ($site_id) {
  if (-d '/path/to/sites/'.$site_id) {
    &error("Directory already exists");
  }
  else {
    # directory doesn't exist, so do whatever...
  }
}
else {
  &error("Invalid Site ID");
}