Well, okay this is mostly Apache config not Perl:

# By default, Mason handles everything
<Directory /var/www-mason>
  SetHandler perl-script
  PerlHandler HTML::Mason::ApacheHandler

  Options Includes FollowSymLinks
  AllowOverride None
  Order allow,deny
  Allow from all
  # whatever else...
</Directory>

# But we don't want bare autohandler or dhandler served by Apache
<LocationMatch "handler$">
  SetHandler perl-script
  PerlInitHandler Apache::Constants::NOT_FOUND
</LocationMatch>

# Nor do we want Mason touching stylesheets, javascript, or images
<Location /css>
  SetHandler default-handler
</Location>
<Location /js>
  SetHandler default-handler
  Options FollowSymLinks Indexes
</Location>
<Location /img>
  SetHandler default-handler
</Location>
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And if there's some directory under Mason control that I want to switch to non-HTML, I can put this in an autohandler if for example I wanted to output JSON:

<%init>;
# get rid of crap from parent autohandler
$m->clear_buffer();

# change content-type
$r->content_type('application/json; charset=utf-8');

# call any dhandlers, etc.
$m->call_next();
$m->flush_buffer();

# but don't go back to the parent autohandler
$m->abort();
</%init>
[download]

And well generally there are other tricks but it depends on what you're doing.