Another cause for the Modification of a read-only value error is using a sort block that dereferences $a and $b, and has gaps.

This is ok:

use strict;
use warnings;

my @list;
$list[0] = 0;
$list[2] = 2;
@list = sort {$a <=> $b} @list;
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This isn't:

use strict;
use warnings;

my @list;
$list[0] = [0];
$list[2] = [2];
@list = sort {$$a[0] <=> $$b[0]} @list;
[download]