You cannot use ACCEPT in PL/SQL.
Is it possible to split your PL/SQL in two blocks, one before and one after the ACCEPT command? (As mentioned by Ad in the Development Forum).

Or you can use a substitution variable, as Wolf mentioned.

If you need the substitution variable (&your_variable) more then one time in your code, use double ampersands (&&your_variable) or asing it once to a variable you use in the rest of your code (my_used_variable := &my_substitition_variable).