Although I'm guessing at what kind of file a ".db" file is (Exim?), it looks like this question is answered in perlfaq5 - and I quote:

  How do I change one line in a file/delete a line in a file/insert a 
+line in the middle of a file/append to the beginning of a file?
    Use the Tie::File module, which is included in the standard distri
+bution since Perl 5.8.0.
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