Modelsim: Force string
If you open the command reference manual by going to 'Help > Documentation - PDF Bookcase', you can find the complete syntax for this command.

It would be helpful if you included the error message and/or problem you saw, as opposed to simply "it didn't work", but in your case, it looks like you have have forgotten to include the full path to the signal you are driving. simulation_input_file should be /dpu_tb/simulation_input_file. In addition, to force a string, your force value must be the same length as the target string. If this is not convenient, you can make use of the TCL format command, for example:

force /dpu_tb/simulation_input_file [format "%-100s" /tmp/test]

You can look up the format command in the online TCL documentation to understand how this works.