my $updateDeduct = $DBH->prepare("UPDATE USER SET BALANCE = BALANCE - ? WHERE USERNAME = ?");
my $updateAdd = $DBH->prepare("UPDATE USER SET BALANCE = BALANCE + ? WHERE USERNAME = ?");

$DBH->do("BEGIN");  #  explicitly start a new transaction
                    #  this overrides autocommit setting

$updateDeduct->execute($amount, $user);
$updateAdd=>execute($amount, $user);

$DBH->do("COMMIT");  #end transaction, this is the "expensive part" timewise