I guess I don't really understand what your asking. There are several modules capable of parsing XML. XML::Parser and XML::LibXML both have external C libraries that need to be compiled and work fine on OS X (check the Perl-XML mailing list to get the details on compiling LibXML). There are also several pure-Perl solutions. XML::SAX was previously mentioned. XML::Parser::Lite, which is included with SOAP::Lite, also works well. I would highly suggest, however, that you use an existing module rather than write your own. For what it sound's like your doing, however, you really would probably want XML::LibXSLT as well to do the conversions from XML to HTML.

However, if this project is something that you are looking to distribute as shareware or freeware of some sort, my guess is that you are thinking in the wrong direction. While I think that it is admirable to try to come up with your own XML Parser and XML to HTML conversion routines in Perl, the two have been done many times over. Writing your own bug free routines would likely be risky at best. In a case like this, I would probably point you in the direction of an all Objective-C program, where you could compile in LibExpat or LibXML, or Java, where you could include whatever Parser and XSLT routines you wish into a JAR file.