Rather than "if an actual list is returned", just say that you want the function to return the last value if used in a scalar context. Then people won't have to try to figure out what lists you consider to be "actual" vs. whatever the other lists are. (:

What "expense" are you complaining about here? I don't see any big CPU or memory cost being incurred in the last case. Perhaps clarity? If so, just write the code less densely. For example, rather than using a slice just to get the side effect of returning the last item in a scalar context (a justification that would require more guessing when your code is later read), make it clear what you wanted:

my %foo_cache;
sub foo {
    my $ret = $foo_cache{join",",@_}; ||= [ do {
        ...
    } ];
    return wantarray ? @$ret : $ret->[-1];
}
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