$debug = 1;
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sub Debug {
    if ( $debug ) {
        my $msg = shift;
        print $msg, "\n";
    }
}
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Debug( "Entering calculation phase." );
Debug( "Pre-calculation value of \$x: $x" );
...
Debug( "Leaving calculation phase." );
Debug( "Post-calculation value of \$x: $x" );
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This will allways call the Debug() sub. The overhead may or may not be a problem in your production systems, it all depends on how many times this call is executed.

Imagine a few thousand times per minute. (Or per second...)

I propose the alternative solution:


$debug=1;
$debug&&print("Debug info...\n");
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The above should require less overhead. I've even read somewhere that Perl optimizes this so that the $debug&& doesn't have to evaluated, but unfortunately I can't find back the reference to that text. :-( Maybe some of the well educated Monks may chime in about that?

If there is no reason to change the value of $debug during execution, the following should be even smarter(?):


*DEBUG=\1;
$DEBUG&&print("Debug info...\n");
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f--k the world!!!!
/dev/world has reached maximal mount count, check forced.

I'm sure your way is more efficient compared to calling a sub all the time, but I would definitely push for "use constant" in this scenario, especially if you're concerned with efficiency

Suppose the following is in a file called "debug.pl":

*DEBUG = \1;
use constant CONST_DEBUG => 1;
$DEBUG && print "foo\n";
CONST_DEBUG && print "bar\n";
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They look pretty much the same, but you should see the output of perl -MO=Deparse debug.pl

## Output of perl -MO=Deparse on my machine with perl 5.6.1:
*DEBUG=\(1);
sub CONST_DEBUG() {
    package constant;
    $scalar;
}
print "foo\n" if $DEBUG; # <- $DEBUG must be evaluated each time
print "bar\n"; # <- there's no evaluation... just straight print()'ing
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There you see that the second option gives you a literal print "bar\n" without needing to evaluate another variable.

This is because of a couple of reasons: 1 - the perl compiler is smart enough to optimize away "real" constants... and 2 - *DEBUG = \1 is NOT a "real" constant. (You can always assign on top of *DEBUG if you so choose to).

*DEBUG=\1;
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would probably be more clear as:

use constant DEBUG => 1;
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-Blake