Why is this dup failing under BSD?

I've been doing the following in order to use flock ie. covert a file
descriptor to a File Handle

my %hash;
my $hash = tie (%hash, 'DB_File', $listsDb, O_RDONLY|O_CREAT, 0644)
or
die "Cannot open $listsDb\n";

my $fd = $hash->fd;
open(DB_FH, "+<&=$fd") or die "dup $!";
flock (DB_FH, LOCK_SH) or die "flock: $!";


under BSD I get an error when I do this.

Can anybody tell me why?

.

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