$/ is the The input record separator, newline by default (see the perlop man page; type perldoc perlop and find it). The code you posted could be re-written like this:

   foreach my $key (sort keys %clean_data) {
     print $key, $/;
     foreach my $data (@{$clean_data{$key}}) { 
    # the hash value $clean_data{$key} is an array reference
    # wrapping it in @{} gives us the array it references.
    # this array contains array references, 
    # so @$data gives us an array
        print "\t", @$data, $/;
     }
   }

- Dotan