However, my testing on Perl 5.8.8 and 5.10 shows that shift @empty returns a single undef, not an empty list
This is actually the documented behaviour, not (just) a quirk of implementation: shift.

In reply to Re^4: Unshift and push inside of map operation. (test) by JadeNB
in thread Unshift and push inside of map operation. by LNEDAD

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