use constant EMPTY => 0;
$bottles{beer} = 99;

sub take_one_down { return --$bottles{beer}; }
sub pass_around {
  print "No more beer\n" and exit($bar)
    if ($bottles{beer} eq EMPTY);
}

print "$bottles{beer} on the wall\n";
while ($bottles{beer}) {
  take_one_down($bottles{beer});
  pass_around($bottles{beer});
  print "$bottles{beer} on the wall\n";
}