It is possible to write a sort function that will produce any arbitrary order your heart desires. However, it might be difficult to write, and the resulting program might not be very efficient. A powerful technique is to transform the keys into something that can be sorted alphabetically. This is much easier to think about, and lets you take advantage of the special key-reduction optimizations that are built into the newer versions of DB_File.

If you know that the catalog numbers never have more than a certain number of digits, you can make them sortable by adding leading zeroes to numbers, like this:

• 9 -> 009
• 123a -> 123a
• C30 -> C030
• C21/22 -> C021/022

...and so on. You can make this transformation with a simple statement like:

$key =~ s/(\d+)/sprintf "%03d", $1/eg;
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Remember to keep a copy of the original key around so you can show it to your users, and they won't ever have to know what you've done.

If you don't have a guarantee about how many digits can be in a catalog number, there are clever solutions... You could, for instance, store the number of digits followed by the actual digits themselves.