Hopefully this plain-text illustration is clear enough to highlight the problem with your second examaple.

system("/bin/mailx -s "hello" john@work.com");
       ^              ^  ^  ^              ^
       |              |  |  |              |
       quote          |  |  quote          unquote
                      |  bareword
                      unquote
[download]

It ought to be a "bareword found where operator expected" error.