my %real_functions = (
  wipe_system => sub { system "rm -rf /" },
);
my %debug_stubs = (
  wipe_system => sub { warn "wiping system (no, not really)\n" },
);

my $funcs = 
    $debug
    ? \%debug_stubs
    : \%real_functions;

$funcs->{'wipe_system'}->();

##</code><code>##

{
    package SystemFunctions::Real;
    sub wipe_system {
        shift; # will be the class name
        system "rm -rf";
    }
}
{
    package SystemFunctions::Debug;
    sub wipe_system {
        shift; # will be the class name
        warn "wiping system (no, not really)\n";
    }
}

my $funcs = 
    $debug
    ? 'SystemFunctions::Real'
    : 'SystemFunctions::Debug';

$funcs->wipe_system();

##</code><code>##

my %dispatch = (
  incr => sub { $x++ },
  decr => sub { $x-- },
);
$dispatch{$op}->();

##</code><code>##

sub incr { shift; $x++ }
sub decr { shift; $x-- }

main->$op();