Why DBI cannot lock mysql tables?

Can anyone help? I am stuck on it for days. My DBI cannot lock mysql
tables, even if the lock table command works well in mysql command
line.
My setup is: Redhat, MySQL 5.0.15, Perl 5.8.0, DBI 1.38, DBD::mysql
3.0002_3

Here is the simple Perl code replays the problem:

#!/usr/bin/perl -w
use strict;
use DBI;

print "-----------------Begin-------------------------\n";

my $dsn = "DBI:mysql:host=localhost;database=test;port=3306";

my $dbh = DBI->connect($dsn, 'dbuser01', 'i901na22',
{ RaiseError => 1,
PrintError => 1,
PrintWarn => 1,
ShowErrorStatement => 1,
TaintIn => 1,
FetchHashKeyName => 'NAME',
});
$dbh->{TraceLevel} = "1";

my $str;
$str = "LOCK TABLES t1 WRITE"; #This doesn't work.
#$str = "Insert t1 (name) values ('my');"; #This works.

my $_current_sth;
$_current_sth = $dbh->prepare($str); #BREAK HERE:
# returns undef when using LOCK TABLES
$_current_sth->execute();

$_current_sth = undef;
$dbh->disconnect;
$dbh = undef;
print "-----------------End-------------------------\n";

.