LIST: 0 | 3 = 3 (number of elements in the array)

That's wrong. For starters, there's no array involved. And before you say "I meant list", there's no list either involved either.

Like I said in my post, "|" operates on scalars, so

my @a = x() | (4,5,6);
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is evaluated as

my @a = scalar(x()) | scalar(4,5,6);
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And keeping in mind that the comma operator returns it's RHS in scalar context,

    my @a = scalar(x()) | scalar(4,5,6);
=== my @a = scalar(x()) | scalar(5,6);
=== my @a = scalar(x()) | scalar(6);
=== my @a = scalar(x()) | 6;
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Perl doesn't even bother keeping the 4 and 5 in the compiled code:

>perl -MO=Deparse -e"my @a = x() | (4,5,6);"
my(@a) = x() | ('???', '???', 6);
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