I need help doing some basic arithmatic on some database values. Initially, I query my table for all product codes and quantity ( using DBI ). Then I display these non-editable values along with two editable fields for entering a value that will be added/subtracted from original quantity in a form (using CGI) so it looks something like this: 


Product Name  |  Quantity  |  Add  |  Subtract

WidgetA       |  10        |       |
.
.
.

What I need to do happen when the form is submitted is take all the input in the add and/or subtract fields and do the appropriate math on the original quantity value, then store the newly calculated quantity back into the table. Can anyone please give me a good example to work with?

##</code><code>##

use CGI;
my $q = CGI->new;

foreach my $parm ($q->param) {
    if ($parm =~ m/^add_(.*)$/) {
        sub adjust_qty($1, $q->param($parm));
    }
    if ($parm =~ m/^sub_(.*)$/) {
        sub adjust_qty($1, -($q->param($parm)));
    }
}

sub adjust_qty {
    my ($key, $qty) = @_;
    my $sql = qq(update table set qty = qty + $qty where key = $key);
    # do you need DBI help here?
}