my $oldsub_ref = \&oldsub;

$oldsub_ref->(2);
print '-' x 10;

*oldsub = \&newsub;

$oldsub_ref->(2);

##</code><code>##

my $sref = \&oldsub;
sub oldsub {
  my $level = shift;
  print "oldsub [$level]";
  $sref->($level - 1) if $level > 0;
}