{
    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();