installing mod_perl should not affect /usr/bin/perl at all.

you can be assured about which copy of perl is build into mod_perl by

1) configuring it so at build time or

2) checking the http://your-server/perl-status (if you have not disabled it).

Now the installation of mod_perl may cause other stuff to run slow because you've bloated the size of your httpd process. Check out the mod perl site for ways to to remedy the bloat problem (two apache servers, one light weight, one mod_perl enabled, proxy between them).

-derby