When in doubt, use B::Deparse.

U:\proj>perl -MO=Deparse -e "print $a + $b"
print $a + $b;
-e syntax OK

U:\proj>perl -MO=Deparse -e "print $a +$b"
print $a $b;
-e syntax OK
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As you can see, perl guesses from the whitespace after print and its first argument that you intended a unary plus, instead of an addition between arguments. It then decided that $a must be an object (like a file handle) for object indirect syntax.

Personally, I wish Deparse was even more clear about that:

U:\proj>perl -MO=Deparse -e "print $a +$b"
$a->print $b;
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