Apparently introspection isn't part of the API

see attributes, Attribute::Handlers

#!/usr/bin/perl --
package Foo; BEGIN {
    use Scalar::Util qw( refaddr );
    our %attrs;
    sub MODIFY_SCALAR_ATTRIBUTES {
        print "Foo::MODIFY_SCALAR_ATTRIBUTES! @_\n";
        my($package, $scalarref, @attrs) = @_;
        $attrs{ refaddr $scalarref } = \@attrs;
        return;
    }
    sub FETCH_SCALAR_ATTRIBUTES {
        print "Foo::FETCH_SCALAR_ATTRIBUTES! @_\n";
        my($package, $scalarref) = @_;
        my $attrs = $attrs{ refaddr $scalarref };
        return $attrs ? @$attrs : ();
    }
    sub new {
        print "Foo::new! @_\n";
        my $class = shift;
        return bless { yo => [@_] }, $class ;
    }
}

use strict; use warnings;
use attributes();
use Data::Dump;

my Foo $bar : WHAT = 24;
my Foo $ey = 42;
my Foo $new :AND :CHEWY = Foo->new( 42 );

print "\n\n";

for my $ref( \$bar, \$ey, \$new ){
    dd $ref;
    dd [ attributes::get($ref) ];
    dd [ attributes::_fetch_attrs($ref) ];
#~     use Devel::Peek;Dump( $ref );
    print "\n\n";
}

dd \%Foo::attrs;
__END__
Foo::MODIFY_SCALAR_ATTRIBUTES! Foo SCALAR(0x9c8284) WHAT
Foo::new! Foo 42
Foo::MODIFY_SCALAR_ATTRIBUTES! Foo SCALAR(0xa3358c) AND CHEWY


\24
Foo::FETCH_SCALAR_ATTRIBUTES! Foo SCALAR(0x9c8284)
["WHAT"]
[]


\42
Foo::FETCH_SCALAR_ATTRIBUTES! Foo SCALAR(0x99aa3c)
[]
[]


\bless({ yo => [42] }, "Foo")
[]
[]


{ 10257028 => ["WHAT"], 10696076 => ["AND", "CHEWY"] }
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