I agree with your analysis. The example given in perlsec is at best confusing. Honestly, I am not convinced it would pass a code review.

• If @temp, $orig_uid and $orig_gid are just there to hold the starting values, why is one an array and the other a set of 2 scalars? I think this is because the $GID contains not only the primary group but also all the secondary groups of the real user too and because it is a dualvar and therefore would unroll when the array were evaluated. I'd much prefer to have had that commented.
• On the last line, there a numeric equality test between the UIDs and a string equality test between the GIDs. Again this is probably because of the potential non-numeric nature of $GID. But this is also uncommented in the code so we can only assume.
• Like you, I don't see why the values of $UID and $GID are explicitly reset. Given the logic, this appears to be a no-op. Unless it is there to get around the "saved UID" - but again if that's the case it really should have a comment to say so.
• The original code in perlsec uses a bareword filehandle which is generally discouraged nowadays.

So, the code could work correctly and maybe each line is there for a purpose but it is very far from clear as written. It would be useful if others more familiar with the internals of all this could chip in and correct any of my suppositions here which might well be wrong as it is not my area of expertise.

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