more specifically (from a development site on my own web server): in the httpd.conf file, inside of a container (either <Location>, <Directory>, or <VirtualHost>) add these lines:

PerlModule       Apache::Reload
PerlInitHandler  Apache::Reload
PerlSetVar       ReloadAll Off
PerlSetVar       ReloadTouchFile /tmp/reload.modules
[download]

the way it works is this: the first time each module is loaded (or if the module is already in memory, used) mod_perl will check the date/time of the given file. if it has changed since the last time it was parsed, the parsed code in memory will be unloaded and the code will be re-parsed from the disk.

the problem is that doing this check for every module, for every request, causes a signifigant slowdown on a busy server.

setting "ReloadAll Off" turns off the global checking, and the check will only be done for modules which contain the line "use Apache::Reload" in their code.

note that this also works for authentication handlers, in addition to standard content-generation handlers. my guess is that it will work with all handlers, regardless of the request phase they handle, but i've only used it with these two types.