From the HTML::Mason site:

Why do Mason tests fail during install?
The user that your web server runs as ("nobody" is a common choice, and the default on many systems), needs to be able to read the Mason build directory. If you are installing via CPAN as root, that directory is likely down root.cpan, which will have restrictive permissions. Try chmod'ding the build directory, or moving to a different (perhaps world-readable) build directory for the install.

Why am I getting segmentation faults (or silently failing on startup)?
There are a few known mod_perl issues that cause segmentation faults or a silent failure on the part of Apache to start itself up. Though not specific to HTML::Mason, they are worth keeping in mind:
* Are you using a dynamically-linked mod_perl?
Unfortunately, DSO mod_perl builds seem to be reliably unstable, especially under heavy traffic. Rebuild Apache with mod_perl linked statically.

How could I ever recommend this state of affairs to anyone, let alone clients?