Why document() couldn't fetch an xml file with $URL_PREFIX?

System

I defined a File datum pointing to a xml on TeamSite:
Its value turns to $URL_PREFIX/path_to_xml_file

Then in the component xsl, using the below code to fetch this file,
document('$URL_PREFIX/path_to_xml_file')
It's resolved to the below path during transformation, but couldn't fetch back the file successfully.
document('/iw/cci/meta/no-injection/iw-mount/default/main/ABC/WORKAREA/Shared/path_to_xml_file')"

If I hardcoded the acutal path on local file system as below, instead of using $URL_PREFIX, it works.
document('/iwmnt/default/main/ABC/WORKAREA/Shared/path_to_xml_file')

But this kind of hardcode is not allowed since during deployment the actual path on file system will change. Anybody can help me out? Why document() couldn't fetch a file with page token, which is supposed to work? Thanks in advance.