How to truncate addition result nbr that has more than 15 digits.

[perl]

Hi,

I have a Perl script that adds numbers grater than 15 digits using this code:
$vol_hash = ($vol_hash + $data[3]);
when I use substr($vol_hash + $data[3]),0,15);

the result displays:
999999999999992 instead of 299999999999999
or 2.99999999999999
this is adding 999999999999990 3 times.

If I try to embed the following Oracle sql in the perl script, it does not display anything:
$vol_rec_tot = $dbh->prepare("select
substr(sum(volume), 0, 15), company_id
from t_volume_summary
where date is null and
element_id <> 'CANCELED'
group by ID");
Running the sql in Oracle I get the expected result. what is the sintax in Perl?
I appreciate your help, Thanks

[perl]

correction:
$vol_rec_tot = $dbh->prepare("select
substr(sum(VOLUME), 0, 15), ID
from t_volume_summary
where DATE is null and
ELEMENT_ID <> 'CANCELED'
group by ID");