If you're that worried about 5 bytes perhaps YAML (and/or Perl) aren't really what you should be using.

$ wc
key1: val1
key2:
  - elem1
  - 2
  - 3
       5       9      39
$ wc
{key1: val1, key2: (elem1, 2, 3)}
       1       6      34
[download]

(Granted your inlined form isn't valid YAML and the non-inline sequence will eat up a couple extra characters per additional item, but the point remains)