//first check to make sure the url looks like a url and the email
// looks like an email

my $domain;
if($form->{email} =~ m/@(.+)$/) { $domain=$1; }
if (($domain) && ($form->{site_url} =~ m/(?<![^.])\Q$domain\E$/ ))  {
   //the domains match
} else {
  //they do not match
}