#! perl -slw
use constant DEBUG => $DEBUG;
use strict;

print 'hi';
DEBUG and print 'debugging';
print 'bye';

__END__
C:\test>t-debug.pl
hi
bye

C:\test>t-debug.pl -DEBUG
hi
debugging
bye

C:\test>perl -MO=Deparse t-debug.pl
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant ('DEBUG', $DEBUG);
use strict 'refs';
print 'hi';
'???';
print 'bye';

C:\test>perl -MO=Deparse t-debug.pl -DEBUG
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant ('DEBUG', $DEBUG);
use strict 'refs';
print 'hi';
print 'debugging';
print 'bye';