4. I want my script to have few dependencies
In addition to Joel's "core competency" argument, there's another reason for CPAN authors to not go overboard with external dependencies: the relentless mathematical certainty that the more dependencies you have, the greater the probability that your users will be unable to install your module cleanly. For example, if you have 100 dependencies and the probability of each one failing its "make test" is 1 in 100, the probability of your module installing cleanly is around (99/100)**100 = 0.37; with only five dependencies, that improves to (99/100)**5 = 0.95.
So, if it's a large, complex domain, such as DBI or XML parsing, I'm delighted to add the external dependency.
But if it's just a "convenience" module, such as File::Slurp or Readonly,
I'll normally make do without it.
To illustrate, notice that both File::Slurp (see "get the tests passing" release by Dave Rolsky after two years of inaction by the module author) and Readonly (under perl 5.10.0) failed their "make test" for over a year.
Update: see also Dependency hell (wikipedia).