use strict;
use warnings;

my @words = qw{bioinfo alpha different};

foreach my $word (@words)
{
    print qq{$word\n};
    my $rhRank = rank($word);
    print qq{$_ -- $rhRank->{$_}\n}
       for split m{}, $word;
}
print qq{\n};

sub rank
{
    my $word = shift;

    my %seen = ();
    my $index = 0;

    my $rhRank =
    {
       map  {$_, ++ $index}
       sort {$b cmp $a}
       grep {! $seen{$_} ++}
       split m{}, $word
    };
}