Managing multiple different sourced programs quickly becomes chaotically complex. It's much better to maintain a single update source for major system components.

Or: Keeping track of when you need to update you own installed version of perl, as well as the system version of perl, is just more work than most people want to deal with.

Also: In case you are the sysadmin, it's fairly easy to accidentally replace the system perl with your new version. Which could have immediate or delayed problems. Immediate if something else in the system is broken by the new version of perl, delayed when your next system update removes the new version and puts the old version back (and then breaks some of your scripts...).

The last one, notably, doesn't apply to CPAN modules generally (there are a very few cases where it does...): If you install something that's not in the system, it won't affect anything in the system, as they will never use it.

And, depending on the situation, the developer may not have control over - or even access to - the deployment environment. Think of the case where they are developing something for a department, which will then be placed on multiple department servers.

Generally, I think of it as best to require as few auxillary software changes as possible when you are developing software. If there is something that really will make a difference in how long it will to get the program written (or if it gets written), use it, but if it would just be 'nice to have', try doing without.

Often that will make a difference in whether you can get approval to use the script you just wrote, or not.