Select Code to Download

use strict;
use warnings;
...
    my @holder_sorted = sort { $a->[0] <=> $b->[0] } @holder;
    print map {"$_->[1] => $_->[0]\n"} @holder_sorted;
}

a1_2 : a1_2 : 0.556
a1_3 : a1_3 : 0.345
...
a3_2 : a3_2 : 0.125
a3_2 => 0.125
a3_1 => 0.345