As perldoc -f sort will tell you, the sort verb in Perl takes a subroutine name or an in-line code block as its argument. That piece of code is handed two variables named $a and $b corresponding to the two items now being compared, and it must return a value that is less than, equal to, or greater than zero. Perl provides a special operator, <=>, specifically to assist with this. Also cmp ... which compares strings. (Do not get tripped-up by this peculiarity of Perl!)
The || operator is also useful because it is a “short-circuit logical-OR.” If the expression on the left side is nonzero (“true”), the right side won't be evaluated. Perfect for chaining comparisons together to sort by more than one field.
For anything but very simple cases, I prefer to define a separate sort-comparison subroutine, because an in-line code block can easily devolve into “chicken scratches.” The code, however you choose to write it, should be extremely obvious, able to be tested in isolation, and also very easily changed. It will be executed tens of thousands of times.
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