"XQUERY" is a command to set a mode in the SQL interpreter SQL*Plus. So it is not send to the backend database engine.
Once you have set XQUERY mode then the following statement ('.....') will be embedded in a SELECT XQUERY statement by SQL*Plus before it is send to the database engine.

So if you would want to do something similar from within Perl you would need to enter in your prepare method a text like this:

SELECT XQUERY('......' RETURNING CONTENT) FROM dual;
[download]

NB: xquery SQL extensions are not supported by all databases