The original version may fail to merge keys if the merged hash is a sibling of the merging hash and itself merges a sibling. Success or failure then depends on the order in which hash values are returned by keys()! 
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
...

[download]
If key2 is pushed onto @_ before key3 all is well, but if not there will still be a merge key pointing at key1 after key2 has been merged into key3. Replacing the check for the existence of a mergekey with a while loop fixes that, because then a new merge will be performed and the outer while loop traversing @_ will not move on while there is still/again a merge key. 
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;
}

[download]

In reply to Re^2: A fix for merge keys in YAML::XS, YAML::Syck by Anonymous Monk
in thread A fix for merge keys in YAML::XS, YAML::Syck by bv

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