C:\test>type junk.pl
#! perl -slw
use strict;
use constant DEBUG => 0;

if( DEBUG ) {
    print "Debugging...";
}

print "Not debugging...";


C:\test>junk.pl
Not debugging...

C:\test>perl -MO=Deparse junk.pl
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant ('DEBUG', 0);
use strict 'refs';
'???';
print 'Not debugging...';
junk.pl syntax OK