I altered the Perl as shown below to get the value of $dbh->errstr.
I then printed this out with the text I used in the $dbh->do statement.
The change code is next.

    print "\n[lock_table] entry\n\n";

    $lock_res = 'unset';

    $lock_str = "LOCK TABLES " . $table . " WRITE";

    $lock_res = $dbh->do($lock_str);

    $dberr = $dbh->errstr;

    print "[lock_table] locking table <$table> with <$lock_str>\nresul
+t <$lock_res>\nerror message <$dberr>\n";
[download]

The output is next

[lock_table] entry

[lock_table] locking table <Stock> with <LOCK TABLES Stock WRITE>
result <>
error message <>
[download]

As you can see the error string did not contain anything.
Any ideas about what to do next?