A slice is not an array. What you return is what you get.

Indeed, but that distinction isn't defined for subroutine return. From perlsub:

The Perl model for function call and return values is simple: all functions are passed as parameters one single flat list of scalars, and all functions likewise return to their caller one single flat list of scalars. Any arrays or hashes in these call and return lists will collapse, losing their identities--but you may always use pass-by-reference instead to avoid this. Both call and return lists may contain as many or as few scalar elements as you'd like. (Often a function without an explicit return statement is called a subroutine, but there's really no difference from Perl's perspective.)

...

A "return" statement may be used to exit a subroutine, optionally specifying the returned value, which will be evaluated in the appropriate context (list, scalar, or void) depending on the context of the subroutine call. If you specify no return value, the subroutine returns an empty list in list context, the undefined value in scalar context, or nothing in void context. If you return one or more aggregates (arrays and hashes), these will be flattened together into one large indistinguishable list.

Subroutines return a list of scalars - that's it. Nothing is said about how flattening of aggregates is done, nor is any distinction made between arrays and plain lists.

I'd not see that as a language feature (muss less a desired one), but as a dark corner which should be inspected for sanity. All of the following snippets should behave the same way:

perl -le 'sub x {@x = qw(a b c); @x            }; $r=x; print $r'
3
perl -le 'sub x {@x = qw(a b c); ()=@x         }; $r=x; print $r'
3
perl -le 'sub x {@x = qw(a b c); @x[0..$#x]    }; $r=x; print $r'
c
perl -le 'sub x {@x = qw(a b c); ()=@x[0..$#x] }; $r=x; print $r'
3
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Any formal explanation why the third and fourth of these (should) yield different output?

Last but not least - allowing a more relaxed coding style in test scripts: IMHO that is the place where most robust code is required: flawed tests are useless.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}