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my $source = Sub::ParamObject->new({
    selector => sub {
...
            and $_[1] =~ qr!^https?://!i
    },
});

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' }, ],
});

my $source = Sub::ParamObject->new(test => {
    selector => {
...
        $_[0]->add_check(selector => need_url => sub { $_[1] ne 'filen
+ame' });
    },
});