I believe a tied hash can override how it behaves in scalar context, so that an empty hash can return true and a non-empty hash can return false.

An overloaded object can do various tricky things with regards to how it behaves as a boolean. For example, it could die.

if ($x || $y) {
  say "Something was true. What was it?";
  say '$x' if $x;
  say '$y' if $y;
}
else {
  say "Neither was true.";
}
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It is possible for this to print "Something was true?" but not tell you what was true. This is because $x or $y could be an overloaded object returning true or false at random each time they are checked.