An easy method for looking up information like this in the future is to "cd" to Perl's Pod directory, which on my windows box is \Perl\lib\Pod. Then use

grep open perl*delta.pod |grep =head
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The above gives

perl561delta.pod:=head2 open() with more than two arguments
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and "perldoc perl561delta" gives you the documentation on changes. A "find" with "/open\(\)", with a few "next"s takes you right to what you want to see.