Mod_perl doesn't feel like a reverse proxy to me, it's just executing Perl in the Apache process directly. Fastcgi, on the other hand, is passing requests to persistently running standalone Perl instances, over a pipe, Unix domain socket, or tcp connection. Which feels more like a reverse proxy.

Mod_perl would more accurately be called a “commonly reverse proxied app server.” Typical setup was to have another Apache instance in front of it handling static requests and reverse proxying app requests.