I'm not quite sure I understand where your confusion lies with cmp... cmp is just an operator that returns -1 if its left operand is alphabetically before its right operand, 0 if the operands are equal, and 1 if the left operand is alphabetically after the right operand.

When you use the sort function of perl, you can tell it how you want it to behave. You don't need to pass the elements of your list to $a and $b, sort will do it for you.

@abclist = qw(a b c d e);
@numlist = qw(1 3 5 7 9);
print sort {$a cmp $b} @abclist; // Default sort, prints 'a b c d e'
print sort {$b cmp $a} @abclist; // Prints 'e d c b a'
print sort {$b <=> $a} @numlist; // Prints '9 7 5 3 1'
print sort {$a == 7 ? -1 : ($b == 7 ? 1 : $a <=> $b)} @numlist;
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That last one is the most interesting. It ensures that 7 will always be the first element in the list... if $a is 7, no matter that $b is, we say that $a is less than $b; if $b is 7, no matter what $a is, we say that $a is greater than $b; otherwise, we do a normal <=> comparison.

Sort allows you to provide a block of code that will evaluate to a negative number, 0, or a positive number to create custom sort behavior.