package BaseClass;
use Class::Std;
{
    sub as_num :NUMERIFY  { return 42 }
#    sub as_num { return 42 }
#    use overload (q{0+} => 'as_num', fallback=>1);
    1;
}

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

use BaseClass ;
my $c = BaseClass->new ;
my $x = 0 ;
$x += $c ;
print "x = $x\n";

##</code><code>##

[stbaldwin@audev02 dev]$ ./tst.pl
Use of uninitialized value in method lookup at (eval 2) line 3.
Can't locate object method "" via package "BaseClass" at (eval 2) line 3