Your code will not produce the same output as the OP's.

As long as grep doesn't change the order of 0..$#allrefs, I'd expect the result to be in the same order as by the OP.

About the performance point, this depends on wether %uni_refs has the same size as @allrefs, or not.
If %uni_refs is just a small part of @allrefs, ikegami's solution is faster even with the additional sort...

Update: Oops, I should take a course in reading benchmarks ... map+grep is slower, anyway.
Yet I'm not completely convinced. There must be an edge case where it is faster :-)

In reply to Re^3: Code Interpretation by soonix
in thread Code Interpretation by Perl_Ally

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