There's no need to unlock the file before closing. On older versions of Perl, this can result in data loss because unlocking can happen while there are unflushed buffers. Newer Perls do a flush before unlocking.

Just truncate the file to zero and close.

For finer-grained control, look at a combination of sysopen with the O_EXCL flag and flock. See perldoc perlopentut (which doesn't seem to be in the Library).