use Carp 'croak';
sub UNIVERSAL::AUTOLOAD {
  my($m) = $UNIVERSAL::AUTOLOAD =~ /::(\w+)$/;

  croak "Undefined subroutine &$UNIVERSAL::AUTOLOAD called"
    unless $m eq 'global';

  print "this sub really is global\n";
}

global();

{
  package foo;
  global();
}

__output__

this sub really is global
this sub really is global

##</code><code>##

sub main::global {
  print "this isn't quite global\n";
}

::global();
{
  package foo;
  ::global();
}

__output__

this isn't quite global
this isn't quite global