A lot of people have already mentioned the precedence, in particular with open. An example implementing both would be:

open FILE, $file || 'default.txt' or die "Can't open: $!";
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If you don't know how something will be parsed, use B::Deparse, and have it add a lot of parenthesis using -p:

perl -MO=Deparse,-p -e'open FILE, $file || "default.txt" or die "Canno
+t open: $!";'
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Which outputs (without color):

(open(FILE, ($file || 'default.txt')) or die("Cannot open: $!"));
-e syntax OK

Hope this helps understanding.

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