Well I discovered the source of the error and, it's pretty dumb on my part. But maybe someone will ask this question again so here goes...

Mandrake runs two Apache servers at once, a regular one on port 80, and a mod_perl enabled one on port 8200.

To use mod_perl dependent programs i'd need to call the script as http://hostname:8200/test.mhtml with my original config, or alternatively configure it under /var/www/perl which is set up to automatically calls the mod_perl enabled httpd-perl at http://hostname/perl/mason/test.mhtml

I used this config in commonhttpd.conf

<IfModule mod_perl.c>
   AddType  text/html  .mhtml
   PerlSetVar MasonCompRoot  /var/www/perl/mason
   PerlSetVar MasonDataDir   /var/www/mason-data
   <FilesMatch "\.mhtml$">
        SetHandler   perl-script
        PerlHandler  HTML::Mason::ApacheHandler
   </FilesMatch>
</IfModule>
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