my $dbh = $db->prepare("SELECT balance FROM Table1 WHERE username=? and balance < '-.01'");
$dbh->execute($authuser);
my ($balance_due) = $dbh->fetchrow_array;

##</code><code>##

$balance_due = -$balance_due

# or

$balance_due = $balance_due * -1;

##</code><code>##

$balance_due =~ s/-//;