Is it possible to only be prompted for a given variable once per script rather than for each reference?
If we're talking script logic to execute in the editor, it's possible, but may be database platform dependent, so check the SQL or procedural language reference from your data source vendor.
For example, in SQL Server, you can declare and introduce variables with the '@' symbol... e.g. @InpValue, and you'll be prompted at run time for the variable value.
If we're talking Toad's Automation Scripts, then use the Set Variable, and Set Variable Value tasks in the System Activities category, per below snap.
I have had no issues via Toad Automation in setting my variables and being asked only once for them. The steps given by Gary have worked for me for months.