You're overthinking this. constant and namespace::clean do play nicely together...

use 5.010;
use strict;
use warnings;
use Test::More;

{
    package Foo;
    use constant XYZ => 123;
    use namespace::clean;
    sub add_xyz { shift; return XYZ + $_[0] };
}

ok( !Foo->can('XYZ'),        'constant was cleaned away' );
ok(  Foo->can('add_xyz'),    'method still exists' );
is(  Foo->add_xyz(100), 223, 'method works properly' );

done_testing;

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The problem with your tests is that you assume that use namespace::clean deletes subs immediately. It does not. It uses B::Hooks::EndOfScope to wipe the subs away when the enclosing scope is finished compiling.

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re: Optimized constants without namespace pollution? constant::co? by tobyink
in thread Optimized constants without namespace pollution? constant::co? by Anonymous Monk

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