When I need to distinguish between the three, I generally go with

  if (!defined($bSomething)) {
    ...
  } elsif ($bSomething) {
    ...
  } else {
    ...
  }
[download]

and sometimes

  my $x = defined($bSomething)
    ? $bSomething : $DEFAULT_VALUE;
  if ($x) {
    ...
  } else {
    ...
  }
[download]

The verbosity doesn't really bother me; it works with older versions of Perl; and it's easy to read.

Best, beth