One approach I have used would be to KEEP the program external as its own "MyScriptTest.prg".

Then in your "MyProc.prg", just have it do a direct call to "DO MyScriptTest"
Yes, you are now getting an extra layer into the call-stack, but you should get full debug because it is a hard program with determinable line numbers to go against.

Once you get the script working, then copy the "MyScriptTest.prg" code back into your "MyProc.prg".