C:\test>perl -MO=Deparse -Mconstant=DEBUG,1 -le"print 'hi'; DEBUG and print 'here'; print 'bye'"
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant (split(/,/, 'DEBUG,1', 0));
print 'hi';
print 'here';    ## When DEBUG is defined; it doesn't need to be tested for at runtime. (No conditional!)
print 'bye';
-e syntax OK

C:\test>perl -MO=Deparse -Mconstant=DEBUG,0 -le"print 'hi'; DEBUG and print 'here'; print 'bye'"
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant (split(/,/, 'DEBUG,0', 0));
print 'hi';
'???';          ## And when it's not; both the conditional and the code it controls are simply optimised away.
print 'bye';
-e syntax OK