Was just working trying to trackback to test trackbacks on another site, and noticed that I was getting an error. A look at the activity log showed the following:
Ping 'TRACKBACK URL' failed: HTTP error: 500 Can't locate HTML/HeadParser.pm in @INC (@INC contains:SOME PATHS
A little research showed that I needed but didn't have the libwww perl library which contains the HeadParser.pm perl module
Since I am on a FreeBSD system it was a simple matter of installing the port:
portinstall -v p5-libwww