Easy enough to test at any rate

And easy enough for you to test to find out that parentheses around the arguments are not a problem. You could also check the docs and notice that all the code examples in perlopentut, as well as the examples of sysopen in the open documentation *all* use parentheses.

Perl is a programming language, not a guessing game. You might as well have asked if the OP walked around his chair three times chanting bits of the Katha Upanishad before running the code. Not a big difference, and easy enough to test right?