I had to add SQL (postgres) :P

            Rate     Span SQL (Pg)     Perl     Pair
Span      8489/s       --     -44%     -56%     -86%
SQL (Pg) 15132/s      78%       --     -22%     -75%
Perl     19388/s     128%      28%       --     -68%
Pair     60727/s     615%     301%     213%       --
[download]

adding:

sub _sql {

  return $dbh->selectrow_arrayref("
select array_to_string( array(
  select lower(r) || '-' || upper(r)-1 from (
    select 
      (select unnest(r1) limit 1 offset n -1) *
      (select unnest(r2) limit 1 offset n -1)   as r
    from (values 
          (('{[' || replace(replace('$TM_part1', ',', '],['), '-', ','
+) || ']}')::int4multirange,
           ('{[' || replace(replace('$TM_part2', ',', '],['), '-', ','
+) || ']}')::int4multirange
          )) as f(r1,r2)
    , lateral generate_series(1, 4) as g(n)
  )       as g
), ',')   as h
;
")->[0] ;

}
[download]

Update: see my next message; it has improved/faster SQL and different Benchmark