my $IP = '127.0.0.1';

my $abs_IP = sprintf "%03d"x4, split /\./, $IP;
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Avoid map for simple stuff. The "%03d"x4 above is computed during compilation. Also, avoid loops at runtime.

You should also consider using the resutling string as a prefix of the resulting filename for security reasons. There is File::Temp for that.

UPDATE: Ignoring the fact that this might not be as useful as one might think, you should really use a database for this type of job. It's a lot safer, faster and scalable.

It depends on what you store in each file. If you store small records of just a few bytes, you could be using 2k or 4k (or more) of disk space for each file according to your fs blocksize. Take a look to DB_File for more info.