use DBI;

$dbh = DBI->connect("dbi:ODBC:DSN=$dsn") or die($DBI::errstr);
$sth = $dbh->prepare("select monto from table producto where pid=1") or die($DBI::errstr);
$sth->execute() or die($DBI::errstr);
while ($rs = $sth->fetchrow_hashref) {
    print "monto = $$rs{'monto'}\n";
}