use strict;
use warnings;
use diagnostics;
use Data::Dumper;

my @buffer = qw[a b c d e f 0 1 2];
my $bufsize = 6;
my $diff = 2 * $bufsize - scalar(@buffer);
my @smallbuf;
my $writes = 0;
my $source = 0;
my $chainfrom = 0;
push @smallbuf, $buffer[$source];
while ($writes < scalar(@buffer)) {
    my $dest = $source < $bufsize ? $source + $bufsize - $diff : $source - $bufsize;
    push @smallbuf, $buffer[$dest];
    $buffer[$dest] = $smallbuf[0];
    $writes++;
    shift @smallbuf;
    if ($dest == $chainfrom) {
        $chainfrom++;
        $source = $chainfrom;
        $smallbuf[0] = $buffer[$chainfrom];
    } else {
        $source = $dest;
    }
}
print Dumper @buffer;