So the answer is "neither - rename closes the filehandle".

I think you missed the bit about "... two concurrent (Perl) processes ..." and "... when the second process next reads via it's open handle ..."

A better test would be

#!/usr/bin/perl
use strict;
use warnings;

open OUT, '>', 'junk.1' or die $!;
print OUT 'X'x 20 . "$_\n" for 1 .. 20;
close OUT;

open OUT, '>', 'junk.2' or die $!;
print OUT 'Y'x 20 . "$_\n" for 1 .. 20;
close OUT;

open IO, '+<', 'junk.1' or die $!;
seek IO, 0, 0;
print scalar <IO>;

system $^X, '-le', q[ print rename( 'junk.2', 'junk.1' ) ? 'worked' : 
+'failed'];

print scalar <IO>;

seek IO, 0, 0;

print scalar <IO>;

close IO;

unlink 'junk.1';
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