my $x;
demonstrate();

$x = 1;
demonstrate();

$x = 0;
demonstrate();

sub demonstrate {
  if (defined $x) { print "\$x has value $x\n"; }
  else { print "\$x is undefined\n"; }

  if ($x) { print "\$x evaluates as TRUE\n"; }
  else { print "\$x evaluates as FALSE\n"; }

  print "\n";
}

##</code><code>##

$x is undefined
$x evaluates as FALSE

$x has value 1
$x evaluates as TRUE

$x has value 0
$x evaluates as FALSE