CrazyInsomniac and others, thanks for your replies.

  I'm using Perl v5.6.0, DB_File v1.73, and DB_HASH.

  I looked at locking and race conditions in other nodes on PerlMonks, but as the file is not being shared with any other programs I don't think that's the issue.

  Sorry, corruption is rather ambiguous; I mean that at a particular point in the file (the 40,309th key) the value looks like a null to Perl. {Thanks YuckFoo for the hint} But, the step that writes the key/value gives no error, hiccup, or other indication that the write to that particular key went bad.
  One thing I didn't think about, though, you have thankfully brought to light. There's a very good chance that the program got interupted 'whilst' using the file.

Many Thanks,
Skinnymofo