If both constants are false, the print and the if are removed at compile-time.

If one or both constants are true, the if is removed at compile-time, leaving the print to execute unconditionally.

By the way, the benefits of this are minuscule. You might save yourself a couple of seconds every 100,000 times you execute it. The time it takes to evaluate the conditional expression is dwarfed by the time it takes to call the function.

Update: 

          Rate   var const
var   765464/s    --  -20%
const 956589/s   25%    --

[download]
use Benchmark qw( cmpthese );

{ package pkg1; use constant DEBUG => 0; our $DEBUG => 0; }
{ package pkg2; use constant DEBUG => 0; our $DEBUG => 0; }

sub const { print "boo" if  pkg1::DEBUG ||  pkg2::DEBUG; }
sub var   { print "boo" if $pkg1::DEBUG || $pkg2::DEBUG; }

cmpthese(-3, {
   const => 'const(); 1;',
   var   => 'var();   1;',
});

[download]

In reply to Re: use constant and the Perl Optimizer by ikegami
in thread use constant and the Perl Optimizer by Outaspace

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.