You're right about the behavior of slices, but it has nothing to do with subroutine returns. See for example:

perl -e"my @a=qw/a b c/; print scalar @a[0..2]"
c
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The issue is that an array (or hash, or list) slice in scalar context returns the last element of the slice. I feel like this should be documented in perldata but a quick read of it just now didn't reveal any such indication.