That's why you should never use $a and $b as variable names.

Personally, I think instead of promoting a blanket ban on using lexical $a & $b -- which 9x% of code can use safely and usefully -- it would be better to change the documentation to show the use of:

my @sorted = sort{ $::a <=> $::b } @array;
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which avoids the problem and clearly marks out the special usage of these variables for sort.

A simple change that would easily fix the problem.