my @toprocess = qw (a1 a2 a3 a4 a5);
my @processed = ();

my $i = 10; # avoid an infinite loop

while (my $item = shift @toprocess) {
    push    @toprocess, 'x' ;
    unshift @processed, 'y' ;
    push    @processed, $item;

    print "array has ". scalar @toprocess. " items left.  This one is $item\n";

    last unless $i--; # break out of infinite loop for this example
}