That's not error checking. His comment meant that you should add error checking there, to make sure the values being compared exist and are numbers, for instance.
The <=> operator (search for it in perlop) compares two numbers and returns -1 if the left argument is less than the right one, 0 if they are equal, and +1 if the left argument is greater than the right. It's very commonly used in sort routines, since sort then uses those -1/0/+1 return values to decide which of two values should come first. The way sort works (simplistic explanation coming) is by comparing values in the list to each other, two at a time, and swapping them if they're out of order, and continuing until the entire list is sorted. The subroutine or block you provide as the sort routine is what it uses to compare the two values, which are passed to that routine as $a and $b.
So he's having sort call sort_italian() to do the comparisons, and it uses the <=> operator to compare the values numerically.
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