test.pl

use Mojo::Base -strict;

use Test::More;
use Mojo::Util qw/slurp spurt/;

package My::Handle {
  use parent 'IO::File';

  use overload '-X' => sub {
    my ($self, $op) = @_;
    if ($op eq 's') { return 0 }
    die "-$op unimplemented";
  };
}

use File::Temp ();
my $tmp = File::Temp->new;
spurt 'lalala' => $tmp->filename;

my $h = My::Handle->new;
$h->open($tmp->filename, '<');
is -s($h), 0, 'correct mock size';

BEGIN { *CORE::GLOBAL::open = sub { $tmp } }

is slurp($tmp->filename), 'lalala', 'correct content';

done_testing;