my $alpha    = 'a-zA-Z';
my $alphanum = $alpha.'\d';
my $any      = $alphanum.'-';
my $label    = qr/
   (?:[$alpha]|                      # start with a letter
      \d[$any]*[$alpha]+)            # or a number if there's a letter elsewhere
   (?: [$any]*                       # followed by more stuff maybe
       [$alphanum]                   # as long as it doesn't end with a -
   )?
/x; 

$is_valid = /^(?:$label\.)*$label$/;