subtype 'OptInt' => as 'Int';
subtype 'OptStr' => as 'Str';

##</code><code>##

use Types::Standard qw(Maybe Str Int OptInt OptStr);

##</code><code>##

has 'xxx'  => (is => 'rw', 
                   isa => (Maybe[OptInt])->plus_coercions(OptInt, sub{ undef }));

##</code><code>##

Could not find sub 'OptInt' to export in package 'Types::Standard' at