Question

How do I define a secondary ordering to the Heap::Simple interface in Perl?

Answer 1

The documentation states that the constructor takes a code reference to define
the order, so you can specify any sort method you like:

my $heap = Heap::Simple->new(order => \&sort_method);

Every time two keys need to be compared, the given code reference will be called like:
$less = $code_reference->($key1, $key2);

This should return a true value if $key1 is smaller than $key2 and a false
value otherwise. $code_reference should imply a total order relation, so it
needs to be transitive.

By “secondary ordering” I assume you mean that a second comparison is used if
the first one shows the values to be equal. Let’s say the first comparison is
of values found via the “method1” method, and the second comparison is of
values from “method2”. So, if by method1 the values are different, return
that result, and otherwise fall back to method2:

sub sort_method
{
    my ($val1, $val2) = @_;

    my $result = ($val1->method1 <=> $val2->method1)
                          ||
                 ($val1->method2 <=> $val2->method2);

    return 1 if $result == -1;
}

If method1 and method2 return strings instead of numeric values, simply use
the cmp operator instead of <=>. You can use anything you like, as long
as the operator returns the right values. Most sort functions like using the
values -1, 0 and 1 to indicate whether value1 is less than, equal to, or
greater than value2, but this module likes 1 to mean val1 < val2, so after
gathering the -1, 0, 1 result, one then returns 1 if the result is -1 (where
value1 is less than value2).