I'm sure it must be feasable to set the value for the constant based on a command line switch. I'm not sure if one can just plug in use a module like Getopt::Std or Getopt::Long for this — they probably work at runtime, and might interfere with your own use of command line switches. Hence the handcrafted code, in the example:

{
   my $debug;
   BEGIN {
       if(grep { $_ eq '-d' } @ARGV) {          # look for switch
           $debug = 1;
           @ARGV = grep { $_ ne '-d' } @ARGV;  # remove switch
       }
   }
   use constant DEBUG => $debug;
}

print "Debug is on" if DEBUG;
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Does anybody else get a "Useless use of a constant in void context" warning for the print line if DEBUG is off, or is it my old Perl version (5.6.1)? -MO=Deparse tells me it got replaced by the statement '???';, which might explain it. It's still silly.