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 [...]

Sun JRE is available in the non-free repository. You need to update your repositories configuration file (/etc/apt/sources.list).
Add the following line:
deb http://ftp.us.debian.org/debian/ etch main contrib non-free
Update apt with apt-get update command and you’re ready to install the JRE.
$ apt-get install sun-java5-jre
Check your java binary:
$ java -version
java version “1.5.0_14″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_14-b03)
Java HotSpot(TM) [...]