Error starting httpd: No root path(s) specified at /usr/local/rt3/bin/ line 98

I have a new installation of RT version 3.4.5 under Debian 3.1 (stable).
The web server is a compiled apache 1.13.34 with mod_perl version 1.29.
There are several perl directories; it wasn’t obvious to me which one is
being used:
apt-get seems to update the first of these. The second was probably created
with an attempt to install mod_perl2 with apache 2.2.0; this was abandoned for
mod_perl 1.29 and apache 1.13.34, since the BestrPractical online documentation
seems to suggest this is the most stable; FastCGI also wanted to refer to the
ap compatibility library; resolving ap_null_handler was a losing proposition.

Initially on startup, http reported that had an error reported elsewhere;
this was resolved with
perl -MCPAN -e 'install CGI::Cookie’
I could see that /usr/local/share/lib/perl/5.8.7 was updated; this also had the ModPerl
subdirectory (these observations aren’t those of a seasoned PERL veteran).

Initially, I had this error when starting httpd:

Starting httpd: No root path(s) specified at /usr/local/rt3/bin/ line 98

This seemed to go away after I set the /perl/ alias in the httpd.conf to

I had a few moments of joy as the RT screens came up without broken links for the first

However, the problem has reared its ugly head again. I have no idea what caused it.
A possible change in one of the PERL installations (I might have done an apt-get of
one of the perl libraries–though my guess is that would have affected the first of the
three PERL directories above)? Only last night I found out about
compiling PERL modules with PREFIX=, after reading; should I do a rebuild with this?
How do I keep a PERL upgrade, which might happen after an "apt-get update; apt-get upgrade"
from undermining hours and hours of work?

More to the point: what do I do to fix this error:

No root path(s) specified at /usr/local/rt3/bin/ line 98