It may have not been there when you started to respond, since I tend to revise and expand my answers after posting, but I did explain the behavior you see. I agree that it is not what most people expect.

The slice is being converted internally into a list: ($x[2]..$x[7]), and then that list is being evaluated in a scalar context, which means that it is not a list, but an expression with multiple comma operators. And the comma operator yields its right-hand side.

Interestingly, defined works the same way, so defined(@x[2..7]) is the same as defined($x[7]). So at least when you undef a hash slice, testing it for definedness works as expected, even though several of the values are defined.

I note again, you cannot provide a list to undef or defined. You get a compiler error. For defined, you can say

defined scalar($x[2],$x[3]) or warn "not defined!";
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and it does the same as it does for a slice. But you can't use scalar() with undef, because scalar doesn't yield lvalues.