@ranks[@sorted_positions] = (0 .. $#sorted_positions);

##</code><code>##

@ranks[1, 3, 5, 7, 9] = (0, 1, 2, 3, 4);

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my @ranks = qw{a b c d e f g h i j}; 
my @sorted_positions = (1, 3, 5, 7, 9);

print "@ranks";
@ranks[@sorted_positions] = (0 .. $#sorted_positions);
print "@ranks";

##</code><code>##

a b c d e f g h i j
a 0 c 1 e 2 g 3 i 4