Prior to CPQ release 23D, it was possible to use the CPQ function urldatabypost, to make CPQ REST GET calls. The API function has since been altered to adhere closely to the standard usage of REST with respect to GET v POST.
Whereas previously a POST request could be used, this has been changed to require that you use the more standard GET functionality. The primary recommendation is to use the urldata function, with the httpMethod parameter set to GET. An alternative is to use the urldatabyget function but this is not preferred over the urldata function.
Please have a look at https://help.bigmachines.com/BMIHelp/Default.htm#BML/FunctionsScripts/URLAccess.htm?Highlight=urldata, for more information on the url access functions.