in reply to How to get xml page source if url is given
You can use any number of HTTP clients to get a URI, wget, curl, GET (from LWP) from the Linux command line. Within a Perl program, as others have said there are also a range of HTTP clients, LWP being the most obvious.
However your question suggests you want the XML source of a page before it is turned into a (X)HTML page. That is more complex as the transformation usually takes place at the server side and you may or may not be able to gain access to that depending on how the server is configured.