Evidently, the // are matching $_, which even though it is undefined is treated as the null string, which happens to match. The \ is also providing a scalar context. Note that the eval is somewhat superfluous, try

perl -e '$_="."; $r=\-d; print $r, "\n", $r + 0, "\n", $$r, "\n";'
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Also, your perl or OS appears to have address space randomization (security feature); some machines return the same numbers every time.

It kinda looks to me like 0+\ might be able to distinguish values depending on how they were arrived at.