In commit 331b2369 I added a test to see what jsonb_plperl would do
with a qr{} result.  Turns out the answer is Perl version dependent.
That fact doesn't bother me particularly, but coping with multiple
result possibilities is way more work than this test seems worth.
So remove it again.

Discussion: https://postgr.es/m/E1f3MMJ-0006bf-B0@gemulon.postgresql.org