my %validation_for = (
  foo => qr/^\d+$/, # foo must be an integer
  bar => \&validate,
}:

sub attr {
  my ( $self, $attr ) = splice @_, 0, 2;

  croak "No such attribute $attr" unless exists $validation_for{$attr};

  return $self->{$attr} unless @_;

  # set the value
  my $validation = $validation_for{$attr};
  my $new_val    = shift;

  if ('Regexp' eq ref $validation) {
    croak "$new_val doesn't match $validation"
      if $new_val !~ $validation;
  }
  else {
    $self->$validation($new_val); # we'll assume it throws an error
  }
  $self->{$attr} = $new_val;
  return $self;
}