Avoid bash's filename expansion when calling `tr`

If you happen to have one or more filenames with a single letter, they
will be expanded and you will end up with a call to `tr [A-Z] a b c z`
for example.

In my case it was:

```
    [carlos@multi git-hub]$ make test
    prove test/
    test/args.t ...... tr: extra operand `a'
    Try `tr --help' for more information.
    Can't find library 'Test::More'. at line 5 in main of test/args.t
    test/args.t ...... Dubious, test returned 1 (wstat 256, 0x100)
    No subtests run
```