Since TRUE and FALSE are barewords in your code, I assume that they are subroutine calls.

Otherwise, they would be caught with use strict (which everyone uses, right?). That is (remembering that use constant defines subroutines),

use constant FALSE => 0;
use constant TRUE => !FALSE;
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Assuming that they return 1 and 0 (or 0 and 1, it doesn't matter), then this would work:

Just use the xor operator:

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

$debug = !$debug; # change 1 to 0 or 0 to 1
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update: added !, explanation/assumption at top.

It's funny. I have been coding in one language or another since the early 80s, and it was only this year that I encountered Yet Another Way to write a flip-flop.

  my $ff = 0;
  while( 1 ) {
    print $ff ? 'hee ' : 'haw ', "\n";
    $ff = 1 - $ff;
  }
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For some perverse reason this really appeals to me, although I refrain from using it in production code. It has a certain mathematical elegance.


--
g r i n d e r

$debug = bungee;

if ($debug eq bungee) {
  $debug = SPELUNK;
}
else {
  $debug = bungee;
}
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There is NO Perl literature that says TRUE is true and FALSE is false; if there is, the author deserves to be spat upon by an angry camel. Use true/false values that everyone else (who knows) does.

if ($debug) {
  # $debug is true
}
else {
  # $debug is false
}

$debug = 1;
$debug = !$debug;  # flip-flop
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package boolean;

use overload (
  '+0' => sub { $_[0][0] },
  fallback => 1,
);

sub new { bless [ $_[1] ? 1 : 0 ], $_[0] }
sub flip { $_[0][0] = 1 - $_[0][0] }
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use boolean;

$debug = boolean->new(1);
print "hi\n" if $debug;
$debug->flip;
print "bye\n" if $debug;
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$debug = $debug ? 0 : 1;

$debug = !$debug;
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sub TRUE {return 1}
sub FALSE {return 0}

# Some assignments to $debug, doing something like
# my $debug = TRUE;

if ($debug == FALSE) {
$debug = TRUE;
} else {
$debug = FALSE;
}
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                 ... sub
                  a   {$
                   _ ^=1}
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sub flip_flop {
  $_ ^= 1;
}
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warning: "Flip Flop" is a trademark of CatFood Entertainment in association Dreamworks SKG and Evil Media Conglomerate Incorporated. All our trademarks are belong to us, not you. Duh.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

$debug = ($debug eq FALSE) ? TRUE : FALSE;
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Update: Yeah but you all use "0" and "1" instead of "TRUE" and "FALSE". That was not my intention.

Update2: Sorry to stretch that blatant things here, but I feel like like a really novice. So in perl 5.7.1 and 5.6.0 $debug == FALSE always evals to "TRUE", while the īeqī operator works ok.

HOWEVER: before I used the use constant FALSE => 0; perl 5.6.0 kept complaining when used "==". No - itīs not too hot here, my computer hasnīt faulty memory nor do have I :-)

Update3: And I hope the last one. First, I would like to thank all of you who responded, youīve pointed me the right way in a fundamental question I was convinced to know the "truth". Well. I went for the use constant solution. My perhaps nitpicking question is:

use strict;
use constant TRUE  => 1;
use constant FALSE => 0;

(use other Modules)
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use constant TRUE  => 1;
use constant FALSE => 0;
use strict;

(use other Modules)
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Ciao

If you're testing on TRUE or FALSE, use ==.
You should check What is true and false in Perl? on the boolean topic.
Basically true is everything except 0, "" (empty string), undef (or statements resulting in any of the previous).
Also check perlop on when to use eq and when to use ==. True values can be tested without actually supplying an eq or == operator.
if ($truevalue) { print 'foo'; } Update:Removed dup info
Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.

If you ever see code that says something like $debug == TRUE, then you need to change that code. Why not write it as: if(  TRUE == ( $debug == TRUE )  ) { or if(  ( ( $debug == TRUE ) == TRUE ) == TRUE  ) { To test for truth, you write: if(  $debug  ) { Most languages create problems for this since there are a ton of "true" values that aren't all == nor eq. But it is even more true in Perl since Perl has multiple "false" values that aren't all eq and that can generate warnings if you use ==.