Perl6::Rules Capturing

saintmike has asked for the wisdom of the Perl Monks concerning the following question:

Fellow monks, I must be doing something incredibly stupid to prevent this simple script from working:

use Perl6::Rules;

grammar Foo {
    rule number { (\d+) { print "Found '$1'\n"; } }
}

"123" =~ m/<Foo.number>/;
[download]

Can you see why it doesn't capture the number but leaves $1 undef'd? Any help apprechiated.