package Foo;

use overload
  '0+'  => \&as_num;

sub new {
  my $class = shift;
  my $self  = { value => shift };
  bless $self, $class;
}

sub as_num {
  my $self = shift;
  return $self->{value};
}

package main;

my $x = new Foo(3);
my $z = (0+$x); # $x+0 also produces error