It might be easier (well, more readable anyway) to break it up into components:

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\.)*$label$/;
[download]

Note that while this matches 1912 hostnames, some newer hostnames have been belatedly declared "valid" though they do not adhere to this RFC (specifically, domains like 411.com or 800.com, which violate the "labels must not be all numbers" rule). Perhaps there is a newer RFC that supercedes 1912?

Perhaps somebody has a better solution...