> you declared the lucky case to be the input

In my defence:

To be sure I also tried

 my @array =  ( ("")x3, (undef)x 3, qw/c b a/, ("")x3, (undef) x3 );

which still gives for cmp_undef1

[
  "",
  "",
  "",
  "",
  "",
  "",
  "a",
  "b",
  "c",
  undef,
  undef,
  undef,
  undef,
  undef,
  undef,
]
[download]

which seems to be another "lucky case".

I don't know how "stable" is defined but it turns out to be not that easy to construct a counterexample without shuffle.

my @array =  ( undef, "", qw/c b a/, "", undef  );

->

["", "a", "b", "c", undef, "", undef]