This is a tough problem, even if you have low level access to the disk. Perl does not. You could write code to delete the file then fill the entire hard disk by creating junk files, but that doesn't sound very efficient! You could perform a defrag following deleting a file and that may confuse the trail somewhat, or it may not.

How secure do you really need to be? There are techniques that may be used to recover data from disks that have been over written (they may require special hardware and likely effectively destroy the hard disk though) so anything trivial you do is unlikely to be enough to prevent highly motivated people from recovering data from your disk.

A good whack with a hammer is likely to be effective however.