It's not that unit tests are bad, they aren't, but for certain applications ... say systems monitoring and GUI apps (not always Perl), it's incredibly hard to write good unit tests.

If you think in layers, and test at interface boundaries, even systems monitoring applications become reasonable to unit test. It's a rather limited slice that actually worries about things like SNMP packets on the wire, and these parts can be stubbed using mock objects.

Quite a few people have pushed through the "it's incredibly hard" barrier and lived to tell that it's simpler than they'd feared, even for GUI tests. Look around a bit, and you'll find lots of good ideas for writing simple tests.