I have a query which currently requires too much time. So I'd like to set a timeout and tried it like this:

my $message= "This message should not appear";
my $exitcode= $UNKNOWN;
eval {
    my $dbh = undef;
    my $sth = undef;
    local $SIG{ALRM} = sub {
        if ($sth) {
            $sth->cancel();
        }
        die $timeoutstr;
    };
    alarm $timeout;

    $dbh = DBI->connect("dbi:Oracle:$sid", $user, $pass,
                         { AutoCommit => 0, RaiseError => 0, PrintErro
+r => 1 } );
    $sth = $dbh->prepare($stm) or do_exit($UNKNOWN, $dbh->errstr);
    $sth->execute() or do_exit($UNKNOWN, $dbh->errstr);
    if(my @row = $sth->fetchrow()) {
        # ...
    }
    else {
        $exitcode= $UNKNOWN;
        $message= "Did not get a result: " . ($DBI::errstr || '');
    }
    $sth->finish();
    $dbh->disconnect();
    alarm 0;
};
if ($@ and $@ eq $timeoutstr) {
    do_exit($CRITICAL, "Timeout after $timeout sec\n");
}
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Unfortunately the alarm handler seems to be not invoked at all until I press Ctrl-C.

Do I make a mistake here? Are there workarounds?