use strict;
my $debug;

BEGIN { $debug = 3; }

use if ($debug > 0), 'warnings'; 
use if ($debug > 1), 'diagnostics';
if     ($debug > 2) { no warnings 'redefine'; }

run_sub();
sub run_sub { print "Running\n";}
sub run_sub { print "Ran\n";}

if ($debug > 3) {print "DEBUG: blah, blah, ...\n";}