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use Fcntl qw(:DEFAULT :flock);
use DB_File;  # demo purposes only; any db is fine
...

tie(%hash, "DB_File", $DBNAME, O_RDWR | O_CREAT)
    or die "can't tie $DBNAME: $!";

untie %hash;    # must close database before lockfile
close DBLOCK;   # safe to let go of lock now