I haven't dug into the BackupPC code to verify the exact method used, but it has to be using either do or eval, because the config files aren't limited to variable assignments. If you hand-edit them, they're also able to run arbitrary perl code.

The overall setup, for those unfamiliar with BackupPC, is that there's a main config.pl file containing a bunch of perl variable assignments representing the global configuration, and then each individual host that's backed up can, optionally, have a [hostname].pl file with additional variable assignments to override the global settings. Both config.pl and the [hostname].pl files can be edited through the CGI interface, which then uses Data::Dumper to overwrite the files with the new values.