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' });
    },
});