Page 181, paragraph 4
$ ruby copy.rb testfile
% ruby -w copy.rb testfile
copy.rb:9:in `gets’: No such file or directory @ rb_sysopen - testfile (Errno::ENOENT)
To make it work, I had to copy testfile from code/tut_threads to code/tut_io.
Suggestion : duplicate testfile in tut_io.