package Form::Validator;

sub new {
   ...
   $self->{allowed_tags} = []; # initially empty
}

sub SetConfig {
   my $self = shift;
   $self->{allowed_tags] = [ @_ ];
}

package WebSite1::FormValidator;

my $fv = new Form::Validator();
$fv->SetConfig(qw(a input ...));
...

package WebSite2::FormValidator;

my $fv = new Form::Validator();
$fv->SetConfig(qw(...));