There's another reason for using the three-arg open. Consider the following statement:

This may look OK but it really isn't, especially if $filename is part of the user input. Consider what happens if $filename is "|rm -rf *". This actually does execute rm -rf *, try it out (well, use echo or something a little safer instead). The three-arg open doesn't suffer from this problem:

If $filename eq "|rm -rf *" it'll try to open a file named "|rm -rf *" (which will probably fail) and won't clobber your directory.

Your code doesn't suffer so much from this problem, because you explicitly tell open how you want the file opened. In any case the three-arg open is vastly preferable.