On some of freshly installed servers (Debian Etch), I encountered these error messages in /var/log/munin/munin-node.log:

Use of uninitialized value in eval {block} exit at /usr/sbin/munin-node line 456, <CHILD> line 8.

What a great error message 🙂

After digging into Google results, I found it was just a problem with host_name variable in the configuration. Default value is hostname.localdomain. I’ve replaced it with a valid hostname and it works!