Couple of things:
You can not use a cmd like that in a skin file. Its a cmd to be executed and is why you get no results.
To show a variable in a label you need to use the format "[HL]=[/HL]$xxxx$" (L,356,489,82,54,162,174,11,8,"Arial","=$myvar$"," LEFT")
Also, the only way to do what your trying to do currently would be to correct the improper variable syntax and use a timer to run the strings cmd so that the variable will be constantly updated.
I have now seen enough users that would like some of the standard lables to also be used w/o the descpription part..ie "mp/h,km/h,%" ect and will be creating a way to specify in the label tip, to not add the desc to it. :)
And using your exact posed code above works, as an executed cmd.