use overload ("0+" => 'value');
sub value {
    my $self = shift;
    exists $self->{'-value'} ? $self->{'-value'} : undef;
}

##</code><code>##

sub gen_id { sprintf "%U", $_[1] }

##</code><code>##

package My;

use overload ('0+' => 'num_conv');
sub new {
    my $self = shift;
    bless {}, ref($self) || $self;
}
sub num_conv { undef }

package main;
my $var = My->new;
print int($var);