Thanks for posting Stevan's gist... I hadn't seen that elsewhere.

When I first saw Ovid's post I could see a MooseX::Declare lightbulb going off in my head ;-)

    use 5.010;
    use MooseX::Declare;

    class Foo {
        use MooseX::Types -declare => [ qw(Filename) ];
        use MooseX::Types::Moose qw(Str);
    
        BEGIN {
            subtype Filename, as Str, where { -x $_ };
        }
    
        method file ( Filename $name ) {
            say "Houston, we have a filename: $name";
        }
    }

    my $foo = Foo->new;
    my $perl = `which perl`; chomp $perl;

    $foo->file( $perl );
    $foo->file( 'no_such_file' );

Above throws the expected exeception on 'no_such_file'.