use Fcntl qw(:DEFAULT :flock);
use SDBM_File;
use strict;
my $db = tie(%h, 'SDBM_File', 'filename', O_RDWR|O_CREAT, 0666)
   or die "Couldn't tie SDBM file 'filename': $!; aborting";
flock($db->fd, LOCK_EX);
#...
undef $db;
untie %h;