This is something usefull. I myself have certain directories to backup at and am always messing with the CD layouts :) Three points I'd do different though:

1. I'd like to see a command line parameter I could use to set the maxsize variable, for sometimes I use DVDs for backing up, sometimes CDRs/
2. I'm a lame guy and like the "-h" (human readable) in most apps. I'd probably like the input/output better stating the size in megabytes, rather than bytes.
3. The output (through Data::Dumper) is ok for testing purposes. If you, however, want to actually use this program's output directly say with "mkisofs", I'd probably want a different output ;)