The compare function is called repeatedly with different elements.

my @numbers = (7, 12, 49, 44);
my @sorted =
    sort {
        print("Comparing $a with $b...\n");
        $a <=> $b
    }
    @numbers;
[download]

Comparing 7 with 12...
Comparing 49 with 44...
Comparing 7 with 44...
Comparing 44 with 12...
[download]

It only does the comparisons it deems necessary. As you can see, 7 was never compared against 49 directly in the example.

Perl currently uses merge sort unless told otherwise.