my @required = qw(name site siteid email email2 pass repass);

my $req_err;
for (@required) {
  next if $q->param($_);
  $req_err++; last;
}
if ($req_err) {
  &error("You forgot some required fields"); 
}
elsif ($q->param('email') ne $q->param('email2')) {
  &error("The two e-mail addresses you entered don't match"); 
}
elsif($q->param('pass') ne $q->param('repass')) {
  &error("Your passwords don't match");
 }
else {
  # do whatever...
}

##</code><code>##

$q->param('siteid') =~ /^\s*(\d+)\s*$/
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");
}

##</code><code>##

my @required = qw(name site siteid email email2 pass repass);

for (@required) {
  next if $q->param($_);
  &error("You forgot some required fields"); 
}

$q->param('email') eq $q->param('email2') 
  or &error("The two e-mail addresses you entered don't match"); 

$q->param('pass') eq $q->param('repass')
  or &error("Your passwords don't match");

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

$site_id or &error("Invalid Site ID");

-d '/path/to/sites/'.$site_id
  or  &error("Directory already exists");

# directory doesn't exist, so do whatever...