for odd length of buffers.

My current understanding is that the logic needs to be different for each of 4 possibilities:

1. Odd length left/even length right buffers;
2. Even length right/odd length left buffers;
3. Even length left/even length right buffers;
4. odd length left/odd length right buffers

Also, your approach seems to do considerable more moving/swapping than is optimal; and Perl's convenient-but-slow:

cmpthese -1, { 
    a => q[ 
        my @a = 1 .. 1e5;
        @a[ $_, $_+2 ] = @a[ $_+2, $_ ] for 0 .. $#a-2 
    ],
    b => q[ 
        my( @a, $t ) = 1 .. 1e5;
        $t = $a[ $_ ], $a[ $_ ] = $a[ $_+2 ], $a[ $_+2 ] = $t for 0 ..
+ $#a-2
    ] 
};;
    Rate    a    b
a 9.65/s   -- -13%
b 11.1/s  15%   --
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array-slice to array-slice assignment, hides a lot of sins.