Parameter cascading variable assignments

I strongly feel that it is NOT possible to cascade variable assignements using user defined variables/parameters (the ones with the double $ at the beginning)

The following two samples fail with an error on recursion.

$$DATE_FORMAT=YYYY-MM-DD
mplt_TEST.$$DATE_FORMAT=$$DATE_FORMAT

mplt_TEST.$$DATE_FORMAT=YYYY-MM-DD
$$DATE_FORMAT=mplt_TEST.$$DATE_FORMAT

The following does not throw an error when variable assignment is carried out but the assignment is done literally.

$$DATE_FORMAT=YYYY-MM-DD
mplt_TEST.$$MPLT_DATE_FORMAT=$$DATE_FORMAT

So the variable/parameter contains the literal string

$$DATE_FORMAT

I, however, could assign "built in" variables/parameters (the ones with the single $ at the beginning) to user defined ones. Sometimes the last part of the "built ins" can be chosen freely why I put built in in quotation marks.