One especially odd factor to consider, is that Perl handles __DATA__ by retaining an open file handle that is used when later read from. This can be quite efficient -- for a single program that contains a single __DATA__ section, the beginning of the __DATA__ section may already be in memory, and no file needs to be open()'d to start reading from it.
__DATA__ sections should rarely or never be used in Perl modules unless you absolutely know what you are doing, and even then, reconsider. If every module had a __DATA__ section, and few of them used it immediately, Perl scripts would quickly use up all available file descriptors on the system. Also, using __DATA__ sections may make the module less portable or harder to store in an alternative format such as PAR. I highly doubt __DATA__ sections work properly for compiled Perl.
Personally, I prefer to store 'data' outside the program, as it allows the data to be accessed easily from multiple sources, and not just the executing (Perl) program.