The attributes documentation is a bit unclear (to me, at least), but as I recall, you need to define custom attributes before you can use them, probably using Attribute::Handlers:

package Canine;
use Attribute::Handlers;

sub Watchful : ATTR(SCALAR) {
    my ($package, $symbol, $referent, $attr, $data) = @_;
    print "Watchful called with $package, $symbol, $referent, $attr, $
+data\n";
}
my Canine $spot : Watchful;

__OUTPUT__
Watchful called with Canine, LEXICAL, SCALAR(0x81606a4), Watchful,
[download]

Also note that handler subs on lexicals get called at run-time, even though most other handlers get called at compile time, and all checking appears to be done at compile time too.

I've never used this construct, though, so I suggest reading the Attribute::Handlers documentation.

J.