1. Two minor cleanups:

    - We don't need to call hv_exists+hv_fetch; we should just check the
      return value of hv_fetch.
    - newSVpv("undef",0) is the string "undef", not a real undef.

2. This should fix the bug Andrew Dunstan described in a recent -hackers
   post. It replaces three bogus "eval_pv(key, 0)" calls with newSVpv,
   and eliminates another redundant hv_exists+hv_fetch pair.

3. plperl_build_tuple_argument builds up a string of Perl code to create
   a hash representing the tuple. This patch creates the hash directly.

4. Another minor cleanup: replace a couple of av_store()s with av_push.

5. Analogous to #3 for plperl_trigger_build_args. This patch removes the
   static sv_add_tuple_value function, which does much the same as two
   other utility functions defined later, and merges the functionality
   into plperl_hash_from_tuple.

I have tested the patches to the best of my limited ability, but I would
appreciate it very much if someone else could review and test them too.

(Thanks to Andrew and David Fetter for their help with some testing.)

Abhijit Menon-Sen