xml file into fixed length file

What do you mean by that? Can you post an example, relevant snippet of XML and desired output format?

In general when you want to transform an XML document into something/anything, the XML-ish way of doing that is with XSLT. Look for example at XML::XSLT or XML::LibXSLT.

Hint: take a look at the XSLT functions: substring(string,start,len), string-length(string)...