From the very top of [perlfunc://open], you'll see the following
- open FILEHANDLE,EXPR
- open FILEHANDLE,MODE,EXPR
The first is commonly referred to as the 2 argument open, whereas the 2nd is commonly known as the 3 argument open.
In your code snippet, the 1st call is an example of the 3 arg open and the 2nd is ...
The docs couldn't be much clearer.
A user level that continues to overstate my experience :-))