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,