It's convenient to mirror the array (switch rows for columns), sort it, then mirror it back.

use strict;
use warnings;

sub mirror {
   my $num_rows = @_;
   my $num_cols = @{$_[0]};

   return (
      map {
         my $col = $_;
         [ map {
            my $row = $_;
            $_[$row][$col]
         } 0..$num_rows-1 ]
      } 0..$num_cols-1
   );
}

my @unsorted_array = ([8,9,10], [6,5,4], [7,8,9]);

my @sorted_array = mirror
                   sort { $a->[1] <=> $b->[1] }
                   mirror
                   @unsorted_array;

require Data::Dumper;
print(Data::Dumper::Dumper(\@sorted_array));

__END__
Original
========
[  8,  9, 10 ]
[  6,  5,  4 ]
[  7,  8,  9 ]

After Bottom Mirror
===================
[  8,  6,  7 ]
[  9,  5,  8 ]
[ 10,  4,  9 ]

After Sort
==========
[ 10,  4,  9 ]
[  9,  5,  8 ]
[  8,  6,  7 ]

After Top Mirror
================
[ 10,  9,  8 ]
[  4,  5,  6 ]
[  9,  8,  7 ]
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