The OWA_UTIL package is part of the web toolkit, used to write web applications to run with mod_plsql. Although there are a number of functions that can be used in a different context, you should really be using them just for that purpose.
You've compiled the procedure, then run it in SQL*Plus, which is not a HTTP server and doesn't use mod_plsql, so it is unlikely to work.
I compiled the following variation on your procedure and called it from a browser via mod_plsql:
create or replace procedure my_proc
l_proc := owa_util.GET_PROCEDURE ;
It worked fine, giving me a web page with "MY_PROC" written on it.
This kind-of proves my assumption that this procedure only works when used in conjunction with mod_plsql and a HTTP server.
Note also, I've set the "l_proc" size to 32767. The "GET_PROCEDURE" function returns a VARCHAR2, so it can theoretically be as big as 32767. You should be careful to size your variables correctly. I'm sure 1024 is fine, but you never know when something will be changed making your code fall in a heap.