You want the if pragma.

Update: Not to mention you'll have problems because the compilation phase use lines would be happening before the runtime initialization of your variable. You'd need to stick that in a BEGIN block.

The problem is that use statements are evaluated before the program is run, so the program can't evaluate the conditional when it needs to figure out whether to use warnings and use Data::Dumper. Consider commenting out those two lines, except when you're debugging. (Or consider always using warnings).

Hays

This is just like the following "C" code:

bool debug = true;  // I think bool is a new C keyword...
if (debug)
{
    #include <assert.h>
    #include "mydebugstuff.h"
}
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You're mixing compile-time and run-time code, then, so it doesn't actually work. Someone mentioned the if pragma - but it's not really needed here.

Simply remove the if's. There's nothing wrong with warnings being used at all times. There's also nothing really wrong with loading modules you don't need - slows down compilation, but not much. If's usage is better suited for modules that may not be there than debugging statements that do no harm if they're left in, IMO. Since Data::Dumper is always installed, it's pretty safe to always load.

That said, if you want to avoid loading Data::Dumper, you can learn how to use require - then you will only load it when you need it - at runtime, not compile-time.

Personally, I tend to 'use warnings' unconditionally in my production code, not only in debug mode. If I was too lazy to stamp out all the warnings before going to production, I deserve whatever happens to me. If something later goes wrong, I like to know about it as early as possible. Just my $0.02 worth.

Update: Blush. I didn't try it until now ... but I get a syntax error if I try to use warnings conditionally:

#!/usr/local/bin/perl
my $debug = 1;
use warnings if ($debug == 1);
use Data::Dumper if ($debug == 1);

syntax error at ./test8.pl line 3, near "use warnings if"
Execution of ./test8.pl aborted due to compilation errors.
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use constant DEBUG => 2;
use if DEBUG==2, warnings;

print ">> $debug $D <<\n\n";
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I'm really known if pragma now, and like it!

20060920 Janitored by Corion: Linkified links as per What shortcuts can I use for linking to other information?

BEGIN {
   use constant DEBUG => 1;
   if (DEBUG) {
      require Data::Dumper;
   }
}
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You forgot to import.

BEGIN {
   use constant DEBUG => 1;
   if (DEBUG) {
      require Data::Dumper;
      Data::Dumper->import() if Data::Dumper->can('import');
   }
}
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