DB_File - flock

This is an attempt to flock a database, a modified version of a snippet of code from the PERL docs at http://www.perl.com/CPAN-local/doc/manual/html/lib/DB_File.html#HINTS_AND_TIPS.

# begin code

use DB_File;
use Fcntl;
# ...
$database = "/home/test/temp.dbm";
$db = tie(%hash, DB_File, $database, O_RDRW|O_CREAT, 0777) || die "Could not tie database!";
$fd = $db->fd;
open(DB_FH, "+<&=$fd") || die "dup ($!)";  # dies here
flock (DB_FH, 1);
# ...
flock (DB_FH, 8);
undef $db;
untie(%hash);
close(DB_FH);

# end code

The error arises from the line: open(DB_FH, "+<&=$fd").  Now, I understand  the concept behind "+<&$fd", but why would this open fail when the tie() did not?  The script terminates without setting $!.

If anyone can answer this I would really appreciate it.  Pardon any typos, its been a long weekend.

Daniel Crowe
Evernight Design