What is the best practice for finding all the superclasses of a Perl class?

Most likely these days you want to use one of the functions from mro, such as mro::get_linear_isa.

use mro;
my @superclasses = mro::get_linear_isa($class);

There is no "standard way" because this is not a standard thing you want to do. For anything other than visualization it is an OO red flag to want to inspect your inheritance tree.

In addition to Class::ISA, there is mro::get_linear_isa(). Both have been in core for a while so they could be considered "standard" for some definition. Both of those show inheritance as a flat list, not a tree, which is useful mostly for deep magic.

The perl5i meta object provides both linear_isa(), like mro (it just calls mro), and ISA() which returns the class' @ISA. It can be used to construct a tree using simple recursion without getting into symbol tables.

use perl5i::2;

func print_isa_tree($class, $depth) {
    $depth ||= 0;

    my $indent = "    " x $depth;
    say $indent, $class;

    for my $super_class ($class->mc->ISA) {
        print_isa_tree($super_class, $depth+1);
    }

    return;
}


my $Class = shift;
$Class->require;

print_isa_tree($Class);

__END__
DBIx::Class
    DBIx::Class::Componentised
        Class::C3::Componentised
    DBIx::Class::AccessorGroup
        Class::Accessor::Grouped

I think Class::ISA is something like you are looking for

use Class::ISA;
use Mojolicious;
print join "\n", Class::ISA::super_path("Mojolicious");

Prints:

Mojo
Mojo::Base

However, it's not some kind of "best practice" since the whole task isn't something Perl programmers do every day.