Sorry, IMHO are you over-complicating.

Your definition of LIST is too narrow , it's not only used for function(LIST) and "comma" is not the only list constructor.

You can find LIST in docs for other cases too.

For me LIST means a piece of code which ...

• is compiled in list context
• returns a list value

nothing more.

for instance map {BLOCK} LIST ...

• map { uc } qw/a b c/
• map { uc } "a" .. "c"
• map { uc } grep {...} ...
• map { uc } "a", "b", "c"
• map { uc } @a

only the last two list constructors allow "list interpolation/flattening", since it's a feature of the "comma operator" which has two variants , and '=>'.

correction: since it's a feature of the naked "list context" w/o operator, what comma does is just propagating the list context down the tree, hence @a=@b,@c is just @a = (@b),(@c)

FWIW: I use "interpolation" primarily for vars in strings like in print "$a $b";

But glossary lists both variants

• interpolation

  The insertion of a scalar or list value somewhere in the middle of another value, such that it appears to have been there all along. In Perl, variable interpolation happens in double-quoted strings and patterns, and list interpolation occurs when constructing the list of values to pass to a list operator or other such construct that takes a LIST.