BrowserUk is spot on. Just by wrapping the expression in parenthesis, you'll get the same results from both functions:

sub c2 {
  my ($a, $b) = @_;
  return ($a and not $b)
}
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I went down a rabbit hole and was playing with Devel::Peek and trying to find clues as to why the returned values were different. Oh, well...

...roboticus

When your only tool is a hammer, all problems look like your thumb.