Also remember that when you set your "$$mystring" variable, that it's a global variable and will be available as long as the application is open (and will contain that same value, unless you reset it somewhere else).
If you aren't going to need that variable value outside of the script you are using, use a script variable (only one dollar sign ... $mystring ) instead. It goes away when the script finishes, which is helpful if you name your variables similarly, or if you duplicate scripts for reuse and forget to comment out or change the variable name.
Sorry for jumping in TStriker ... just thought I'd add my $0.02.