Using $UserHeader lets you insert custom HTML code into the HEAD of your renderede HTML.
E.g. if you use this :
$UserHeader = <<EOH; <meta name="GENERATOR" content="jEdit on Mac OS X" /> <meta name="GENERATOR" content="nano on Gentoo Linux" />
EOH
Your HEAD section will end with something like this :
<HEAD>
<TITLE>Bit-Man @ Home: HomePage</TITLE>
<META NAME='KEYWORDS' CONTENT='Home, Page'/>
<LINK REL="stylesheet" HREF="/wiki.css">
<meta name="GENERATOR" content="jEdit on Mac OS X" />
<meta name="GENERATOR" content="nano on Gentoo Linux" />
</HEAD>