For this kind of thing B::Deparse can be a big help.

perl -MO=Deparse,-p -e 'sub x {return (1,2)} @c=x()||(1,2);@d=x() or (
+1,2);'
sub x {
    return(1, 2);
}
(@c = (x() || (1, 2)));
((@d = x()) or ('???', '???'));
[download]

You can see here how the precedence difference between || and or make a difference in what happens.