While this is nice and convenient for those cases where you have independently valid parameters, it's impossible (or dangerous, you choose) to have pairs (or more) of parameters that are valid together :

  my $ratio = Sub::ParamObject->new({
    nominator   => qr/^[-+]?\d+$/,
    denominator => sub { $_[1] =~ qr/^\d+$/ and $_[1] != 0 },
  });

  # works and is convenient
  $ratio->nominator(-1);
  $ratio->denominator(1);

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

  # and now ???
  # I want to have selector set to one of the values
  # and (only) the corresponding field set.
  $source->selector('filename')->filename('foo.txt'); # works
  $source->filename('foo.txt')->selector('filename'); # cannot in my i
+mplementation
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Either you make the approach dependent on the order of parameters (plausible but ugly IMO), or you find another approach to validation (which I would rather welcome, since I did stuff like this years ago and didn't find a good solution to this).

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
[download]