The comment about style merely related to calling open() but not calling close(). In your script, it's really not a problem - each subsequent call to open() will close the previously opened file since the same filehandle is used each time. The last filehandle will be closed when the script exits.

IO::File allows you to store your filehandle in a normal scalar - which is especially useful if you want to pass it to a subroutine. You would typically use a lexically scoped scalar (one declared with 'my') so that the filehandle was not global.

Regardless of which form of open you use, as tall_man said, you should always check the return value and at the very least call die "$!" if it fails.