I experimented, and the following

{
   local $sth->{TraceLevel} = "2|SQL";
   $rv = $sth->execute(@args)
     or die $sth->errstr;
}
[download]

Produced the desired output, unfortunately surrounded by some verbose lines (see second paragraph). Reducing the trace level eliminated the information.

    DBI::st=HASH(0x2910b18) trace level set to 0x100/2 (DBI @ 0x0/0) i
+n DBI 1.623-ithread (pid 7420)
    -> execute for DBD::mysql::st (DBI::st=HASH(0x2910ab8)~0x2910b18 '
+xLangsdorf') thr#309f18
   Called: dbd_bind_ph
 -> dbd_st_execute for 02497600
    >- dbd_st_free_result_sets
    <- dbd_st_free_result_sets RC -1
    <- dbd_st_free_result_sets
mysql_st_internal_execute MYSQL_VERSION_ID 50147
>parse_params statement SELECT *
   FROM ksr_bf_utf8.t_user_portal
   WHERE f_client = ?

Binding parameters: SELECT *
   FROM ksr_bf_utf8.t_user_portal
   WHERE f_client = 'xLangsdorf'

 <- dbd_st_execute returning imp_sth->row_num 19
    <- execute= ( 19 ) [1 items] at placeholder.pl line 57
    -> STORE for DBD::mysql::st (DBI::st=HASH(0x2910b18)~INNER 'TraceL
+evel' 0) thr#309f18
        -> dbd_st_STORE_attrib for 02497600, key TraceLevel
        <- dbd_st_STORE_attrib for 02497600, result 0
    <- STORE= ( 1 ) [1 items] at placeholder.pl line 56
[download]

So I'd need to redirect the trace to a variable and parse the output for a paragraph starting with 'Binding parameters: ' in case of error...

something like

   open my $trace_fh, ">", \ my $trace_out;
   
{
   $sth->trace("2|SQL", $trace_fh );
   $rv = $sth->execute(@args)
     or die parse($trace_out) . "\n" .$sth->errstr;
}
[download]

Thanks again! :)