It appears this has been implemented in YAML-Tiny.
$ cat merge.yaml
foo: 1
bar: 2
<<:
baz: 3
$ perl -MYAML::Tiny -MFile::Slurp -e'print Dump Load(scalar read_file "merge.yaml")'
---
<<:
baz: 3
bar: 2
foo: 1
Closing ticket.
Thank you very much.
Jim Keenan
Fri Sep 27 23:36:14 2013The RT System itself - Status changed from 'new' to 'open'
Fri Sep 27 23:36:15 2013jkeenan [...] cpan.org - Status changed from 'open' to 'resolved'
Fri Sep 27 23:36:15 2013jkeenan [...] cpan.org - Taken