xml file:
<global_status>
......
 <status name="Uptime" value="816078" value_var="30"/>
</global_status>

code:
    my $now_value = scalar($_->[1]);
    my $last_value = $last_status_data->{value};
    $value_var = $now_value - $last_value;

##</code><code>##

    my $last_value = $last_status_data->{value}+0;