why would a generic Perl program for parsing multiple record data files be useful?

I don't accept the premise that it would be useful. Perl already has built-in functionality to handle fixed-width records ($/ = \$width;) and sentinel-terminated records ($/ = $sentinel;), so it would be redundant. Anything else wouldn't be generic.

how does it work?

(That should be "how would it work?")

Quite well if I wrote it :)