For @list (where you're using |), remember that Perl's | operator is the bitwise or operator, e.g., 1 | 2 results in the value '3'. You probably do understand this one, it's just that you're temporarily confused.

For the difference between the other two, checking¹ perlop will yield this nugget "Using 'or' for assignment is unlikely to do what you want; see below."

¹ I'm using Strawberry Perl 5.10.0