my @a = qw( a b c d e f g );
sub f {
	my $i = shift;  
	print join(',',  @a[$i-2, $i-1, $i]), "\n";
}

f(0);
f(1);
f(2);
f(6);

##</code><code>##

f,g,a
g,a,b
a,b,c
e,f,g