That doesn't look right: some statement handle is being destroyed after prepare returns and before execute is called. Could you show us the actual code you ran to get that trace?

For comparison, when I do this:

my $dbh = ...;
$req = "INSERT INTO log
(`timestamp`,`alert`,`id_daemon`,`id_event_type`)
VALUES
(NOW(),'y','31','3')";
DBI->trace(4);
$query = $dbh->prepare($req);
$query->execute();
DBI->trace(0);
[download]

I get this trace:

    DBI 1.53-ithread default trace level set to 0x0/4 (pid 716)
    -> prepare for DBD::mysql::db (DBI::db=HASH(0x7804d0)~0x8fc970 'IN
+SERT INTO log
(`timestamp`,`alert`,`id_daemon`,`id_event_type`)
VALUES
(NOW(),'y','31','3')') thr#603010
    New DBI::st (for DBD::mysql::st, parent=DBI::db=HASH(0x8fc970), id
+=)
    dbih_setup_handle(DBI::st=HASH(0x8fcbf0)=>DBI::st=HASH(0x60a480), 
+DBD::mysql::st, 8fcc00, Null!)
    dbih_make_com(DBI::db=HASH(0x8fc970), 8fd720, DBD::mysql::st, 440,
+ 0) thr#603010
dbd_st_prepare calling count_params (counting params emulation)
    <- prepare= DBI::st=HASH(0x8fcbf0) at - line 7
    -> execute for DBD::mysql::st (DBI::st=HASH(0x8fcbf0)~0x60a480) th
+r#603010
    -> dbd_st_execute for 008fcd00
---> parse_params with statement INSERT INTO log
(`timestamp`,`alert`,`id_daemon`,`id_event_type`)
VALUES
(NOW(),'y','31','3') num params 0
mysql_st_internal_execute
    <- dbd_st_execute returning imp_sth->row_num 1
    <- execute= 1 at - line 8
[download]