Nice. The only thing I really want to change about that is that double map on $row. I'm thinking that I'd really like to alter the arrays in place instead of making a new copy to (hopefully) keep the memory requirements lower.

# Jeffa
   rows => [
      map {{ cols => [ 
         map {
            { data => defined $_ ? encode_entities $_ : ' ' }
         } @$_ 
      ] }} @$row
   ],

# becomes -{diotalevi}->
my ($row, $value);
for $row (@$rows)
  {
    for $value(@$row)
      {
        $value = { data => defined($value)
                           ? encode_entities($value)
                           : ' ' };
      }
    $row = { cols => $row };
  }

# and later
   rows => \@rows
[download]

Update 0: I initially forgot to create the hash and array refs inline. My bad
Update 1: $row = { cols => $row } used to be $row = [ cols => $row ]
Update 2: rows => $rows was rows => \@rows

__SIG__
printf "You are here %08x\n", unpack "L!", unpack "P4", pack "L!", B::svref_2object(sub{})->OUTSIDE