I don't have an answer. It's just one more example of perl getting confused by ambiguous syntax. For some reason that I don't understand, when parsing the third print statement, perl chooses to interpret $a as a filehandle (which is not open) and the + as a unary operator. But adding a + in front of the $a as well clears up that confusion.

% perl -MO=Deparse,-p -e 'print $a + $b;print $a+$b;print $a +$b;print
+ +$a +$b'
print(($a + $b));
print(($a + $b));
print($a $b);
print(($a + $b));
-e syntax OK
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