G'day carlriz,

Let's say @sorted_positions has five elements (1, 3, 5, 7, 9). (So, $#sorted_positions will be 4.)

The code you posted:

@ranks[@sorted_positions] = (0 .. $#sorted_positions);
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will be evaluated like this:

@ranks[1, 3, 5, 7, 9] = (0, 1, 2, 3, 4);
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The element $ranks[1] will be set to 0, the element $ranks[3] will be set to 1 and so on.

Here's a short piece of code demonstrating this.

#!/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";
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Output:

a b c d e f g h i j
a 0 c 1 e 2 g 3 i 4
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The elements with indices 0, 2, 4, 6 and 8 are unchanged. The remaining elements (with indices 1, 3, 5, 7 and 9) are set to the values 0, 1, 2, 3 and 4 respectively.