my $source = Sub::ParamObject->new({
    selector => sub {
        my $self = shift;
        return unless $_[0] =~ /^(filename|url)$/;
        return if @$self{qw(filename url)} and !exists $self->{$1};
    },
    filename => sub {
        my $self = shift;
        return
          (!exists $self->{selector} or $self->{selector} eq 'filename')
            and $_[1] =~ qr/^\w+$/;
    },
    url => sub {
        my $self = shift;
        return
            (!exists $self->{selector} or $self->{selector} eq 'url')
            and $_[1] =~ qr!^https?://!i
    },
});