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?
}