Hook::LexWrap appears to autovivify wrappers to undef in lines like:

  # Hook/LexWrap.pm:43
                    () = $wrapper{post}->(@_, $return)
                              if $wrapper{post};
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It looks like if exists $wrapper{'post'}; should be preferred.

Perhaps from an aesthetic point-of-view, but it makes absolutely no difference in terms of behaviour. Testing $wrapper{'post'} in an if will not autovivify it. And even if it did, it would autovivify to undef, so the if would still fail, preventing the undefined value from ever actually being used as a subroutine reference.

I can imagine that undef coderefs might confuse it.

Quite probably. But only if they were actually ever called, which the if $wrapper{'post'}; ensures they never will be.