Variables are not substituted in single quotes ('). You can use joining operator (.) to join strings together, like this:

'<?xml version="1.0" encoding ="UTF-8"?><!DOCTYPE drl SYSTEM "http://1
+72.XX.XXX.XXX:40500/lab/api/reserveRequest.dtd"><drl mode="normal" id
+="'.$Id.'"><id="1009"><attributes><attribute name="Clust" link="true"
+ category="INV"><value>8</value></attribute></attributes></drl>'
[download]

You may also want to use some XML generator to generate valid XML in case $id contains special characters, or read about quoting operators.

# I suggest you to buy a Perl book ("Learning Perl" is very good, for example). Your questions show that there is much room for improvement of your Perl skills.