Ah, yes. That helped me find the relevant section in perldata:

If you evaluate an array in scalar context, it returns the length of the array. (Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.)

And it explains why:

@a=(0..9);
@b=(4..6);
$a[@b ]= qw/a b c/;
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behaves differently. It's subtle (at least to me). The LIST subscript gets the comma operator applied, but the ARRAY subscript gets the array length applied.