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
    },
});

##</code><code>##

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

##</code><code>##

my $source = Sub::ParamObject->new(test => {
    selector => {
        value => qr/^(filename|url)$/,
    },
    filename => {
         value => qr/^\w+$/,
    },
    url => {
         value => qr!^https?://!i,
    },
}, if_valid => {
    selector => sub {
        $_[0]->add_check($_[1] eq 'url' ? 'filename' : 'url', forbidden =>  sub { return });
    },
    filename => sub {
        $_[0]->add_check(selector => need_filename => sub { $_[1] ne 'url' });
    },
    url => sub {
        $_[0]->add_check(selector => need_url => sub { $_[1] ne 'filename' });
    },
});