##Object:

sub _byCode {
   $a->asStringFmt() cmp $b->asStringFmt;
}

my $callback = $self->{sortCallback} || "_byCode";
...
foreach my $key (sort $callback @{$myList->getKeys}) {
...

}

##</code><code>##

##Client:

my $byCodeAndX = sub {
  ....
};

my $obj = new Obj( sortCallback => $byCodeAndX )