Are your values in @myarray unique? This has a direct impact since the simplest way to maintain a mapping in Perl is using a hash where keys must be unique. If they are, you can accomplish all you have above with:

#!/usr/bin/perl
use strict; 
use warnings;

my @myarray = (2.5 ,4.8, 3.2, 8.5, 8.2, 12.5);
my %value_map;

my $counter = @myarray;
for my $key (sort {$b <=> $a} @myarray) {
    $value_map{$key} = $key*6/$counter--;
}

for my $key (@myarray) {
    print "$key\t-> $value_map{$key}\n";
}
[download]

Note I have not had to cache any intermediate results - you certainly could if you wanted too, so long as you do not change @myarray.

If the values are not unique, I think the most prudent approach is using two hashes keyed equivalently, probably with 1 .. @myarray.If your array values are not guaranteed unique, sorted indices as per BrowserUK's suggestion is a stronger choice.