my $obj = 'Parent'->new;
$obj->populate(@data);
for my $child_class (@possible_child_classes) {
    if (my $new = $child_class->new_from_parent($obj)) {
        $obj = $new;
        last
    }
}
# Don't forget to handle the case that $obj hasn't changed.

##</code><code>##

sub new_from_parent { die "Should be overriden.\n" }

sub rebless {
    my $self = shift;
    my @possible_child_classes = @_;
    for my $child_class (@possible_child_classes) {
        # etc. using $self instead of $obj.

##</code><code>##

my $obj = 'Parent'->new;
$obj->populate(@data);
$obj->rebless(@possible_child_classes);