use constant FALSE => 0;
use constant TRUE => !FALSE;

##</code><code>##

my $debug = FALSE; # or 0
$debug ^= 1; # now it's TRUE, or 1
$debug ^= 1; # now it's FALSE, or 0

##</code><code>##

$debug = !$debug; # change 1 to 0 or 0 to 1