It's an inconsistency in Perl's implementation of that warning resulting from how Perl decides whether it's looking at a list or not. Strictly speaking the code is ambiguous; do you want my $this, $that, or open my ( $fh ), $filename? You know the answer, and Perl runs it right, but the warning is generated. It's documented here: https://rt.perl.org/rt3/Public/Bug/Display.html?id=7250. It's been around for awhile. You can always wrap parens around the my declaration, like this:

It sometimes (maybe always) goes away if you put "or die $!;" after your open call, so it was less apparent before the days of autodie.