Stuff that gets optimized away does not necessarily take no time at all. They just should take less time than the unoptimized version in the general case (and on my system not optimizing is actually faster than optimizing for this code, which frankly surprised me). See the code below.

Your example uses extremely fast constructs; the difference between doing a single multiplication and one "no-op" vs doing two multiplications (while still doing a relatively expensive function call each iteration for both constructs) is not very informative or useful. 

use strict;
use warnings;
use Benchmark qw( cmpthese );
use constant DEBUG => 0;


my $debug = 0;
my $b = 100;
cmpthese -10, {
    constant   => sub { my $a = 10 * $b; $a = 10 * $b if DEBUG; },
    noconstant => sub { my $a = 10 * $b; },
    var => sub {  my $a = 10 * $b; $a = 10 * $b if $debug; },
};

[download]
Update: as an aside, a really hard-core optimizer would eliminate ALL of the code in these examples.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: use constant and Compiler Optimizations by Joost
in thread use constant and Compiler Optimizations by jbisbee

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