key1: &key1
a: a
b: b
c: c
key2: &key2
<<: *key1
d: d
e: e
f: f
key3:
<<: *key2
g: g
h: h
i: i
...

##</code><code>##

sub mergekeys_loop {
    my ( $orig ) = @_;
    while ( my $ref = shift ) {
        my $type = ref $ref;
        if ( $type eq 'HASH' ) {
            # my $tmphref = $ref->{'<<'};
            # if ( $tmphref ) {
            while ( my $tmphref = $ref->{'<<'} ) {
                die "Merge key does not support merging non-hashmaps"
                unless ( ref $tmphref eq 'HASH' );
                my %tmphash = %$tmphref;
                delete $ref->{'<<'};
                %$ref = ( %tmphash, %$ref );
            }
            push @_, grep { ref eq 'HASH' or ref eq 'ARRAY' } values %$ref;
        }
        elsif ( $type eq 'ARRAY' ) {
            push @_, grep { ref eq 'HASH' or ref eq 'ARRAY' } @$ref;
        }
    }
    return $orig;
}