(After more carefully reading the problem, I'm only going to suggest the object member solution.)

You might also consider making the configuration an object member instead of a class member. Then you could have multiple, differently configured Form::Validator objects in existence at the same time. Here's how it would work:

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(...));
[download]

You could also make the configuration a constructor parameter.